CHAPTER 1 INTRODUCTION
SPECIAL PROPERTIES AFFECTING LIFE IN THE SEA
CLASSIFICATIONS OF MARINE ENVIRONMENTS AND MARINE ORGANISMS
BASIC ECOLOGICAL TERMS AND CONCEPTS
r- and K-selection
THE HISTORICAL DEVELOPMENT OF BIOLOGICAL OCEANOGRAPHY
CHAPTER 2 THE ABIOTIC ENVIRONMENT
Radiation at the sea surface
Radiation in the sea
Sea surface temperatures
Vertical temperature distribution
Range and distribution of salinity
Biological importance of salinity
Biological significance of currents
CHAPTER 3 PHYTOPLANKTON AND PRIMARY PRODUCTION
PHOTOSYNTHESIS AND PRIMARY PRODUCTION
Methods of measuring biomass and primary production
RADIATION AND PHOTOSYNTHESIS
THE EFFECT OF NUTRIENTS ON GROWTH RATE
PHYSICAL CONTROLS OF PRIMARY PRODUCTION
Oceanic gyres and rings
Continental convergence and divergence
Planetary frontal systems
Island mass effect and Langmuir frontal zones
GLOBAL PHYTOPLANKTON PRODUCTIVITY
CHAPTER 4 ZOOPLANKTON
HOLOPLANKTON: SYSTEMATICS AND BIOLOGY
DIEL VERTICAL MIGRATION
SEASONAL VERTICAL MIGRATIONS
ZOOGEOGRAPHY OF THE HOLOPLANKTON
LONG-TERM CHANGES IN ZOOPLANKTON COMMUNITY STRUCTURE
CHAPTER 5 ENERGY FLOW AND MINERAL CYCLING
FOOD CHAINS AND ENERGY TRANSFER
The microbial loop
MEASURING SECONDARY PRODUCTION
Experimental biological oceanography
A COMPARISON OF MARINE AND TERRESTRIAL PRODUCTION OF ORGANIC MATERIAL
CHAPTER 6 NEKTON AND FISHERIES OCEANOGRAPHY
FISHERIES AND FISHERIES OCEANOGRAPHY
World fish catch and fisheries management
Fluctuations in the abundance of fish stocks
Regulation of recruitment and growth in fish
Fishing and the use of near real-time oceanographic data
CHAPTER 7 BENTHOS
Measurements of benthic primary production
Systematics and biology
Sampling and production measurements
DETERMINANTS OF BENTHIC COMMUNITY STRUCTURE
CHAPTER 8 BENTHIC COMMUNITIES
Environmental conditions and adaptations of intertidal organisms
ROCKY INTERTIDAL SHORES
Trophic relations and the rold of grazing and predation in determining community structure
Distribution and limiting factors
Nutrition and producation in reefs
Formation and growth of reefs
Zonation patterns on reefs
What are mangroves
Ecological features of mangrove swamps
Importance and uses of mangroves
Rates of biological processes
HYDROTHERMAL VENTS AND COLD SEEPS
Shallow vents and cold seeps
Unique environmental features of sulphide communities
CHAPTER 9 HUMAN IMPACTS ON MARINE BIOTA
Pesticides and other biologically active organic compounds
INTRODUCTIONS AND TRANSFERS OF MARINE ORGANISMS
IMPACTS ON SPECIFIC MARINE ENVIRONMENTS
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The marine environment provides about 300 times more inhabitable space for living organisms than that provided by land and freshwater combined. All known phyla of plants and animals originated in the sea, and there are presently more phyla represented in the oceans than on land.
In comparison to life in air, the fluid nature of the ocean provides a buoyant environment in which the effects of gravity on living organisms are reduced. Because of this, marine organisms do not have to invest energy in building large proportions of skeletal material, and they expend comparatively little energy in maintaining buoyancy and in locomotion.
Plant growth in the ocean is limited to the near-surface regions because light does not penetrate very far in seawater, and it is further limited by the low concentrations of essential nutrients (eg nitrate and phosphate) that are present at these depths. Because almost all life in the sea depends directly or indirectly on plants, the total plant production at the surface determines the amount of animals that can be produced.
Vertical gradients in environmental parameters (eg light, temperature, pressure) establish depth ranges with distinctive environmental characteristics.
Despite the vast extent of the marine environment, only 2% of the human diet comes from marine resources. However, this represents 20% of the high-quality animal protein consumed by humans.
The benthic environment encompasses the seafloor, and those species of plants and animals that live on or within the seabed form the benthos. The pelagic environment is that of the water column, from the sea surface to the waters immediately above the seafloor; inshore waters form the neritic zone and offshore waters form the oceanic region. Plankton and nekton inhabit the pelagic environment; the distinction between the two groups of organisms is based on relative swimming ability, with nektonic species being stronger swimmers that are able to move independently of current direction.
Pelagic organisms can be classified into size categories ranging from femtoplankton (viruses) through intermediate sizes to the largest nekton (whales).
On an ecological scale, organisms can be considered individually or in assemblages that include populations of a single species, or communities made up of th epopulations of many interacting species. The highest unit of ecological integration is the ecosystem, which encompasses one or more communities as well as surrounding environment.
The life history patterns of all species for a continuum that ranges between the extremes described by r-selection and K-selection. Opportunistic species are adapted to live in variable or transitory environments by having short life cycles, production of many young, and high dispersal ability; however, these r-selected species have high mortality rates and their populations are often of short duration. K-selected species live in stable environments and usually have population densities near the carrying capacity of the environment; these equilibrium species typically have longer life spans, produce relatively few young, and have comparatively low death rates.
Edward Forbes (1815-54) is regarded as the founding father of biological oceanography and the Challenger Expedition of 1872-76 marks the beginning of systematic oceanographic studies that integrate physical phenomena, water chemistry and biology.
New techniques developed in the mide- to late- 1900s expanded the scope and scale of oceanographic research. These include sonar, submarines, scuba diving, underwater sound recording, and remote sensing from satellites, all of which are now used to investigate life in the sea.
The amount of sunlight arriving at the sea surface varies with time of day, season, and weather. Approximately 50% of the solar radiation penetrating the sea surface is within the visible spectrum (about 400-700 nm), and these are spproximately the same wavelengths used in plant photosynthesis. The intensity of photosynthetically active radiation (PAR) at the sea surface ranges from zero (in darkness) to about 2000 μE m-2 s-1 with the Sun directly overhead.
Different wavelengths of light are absorbed and scattered at different depths in water, and they have different extinction coefficients, with red light being attenuated most rapidly and blue light penetrating deepest in clear water. The depth to which any wavelength penetrates depends partly on the amount of suspended particles and chlorophyll in the water.
Three ecological zones have been defined, based on the penetration of light in seawater. The euphotic zone is that region where light is sufficient for the growth of plants, and it extends from the surface to a maximum of about 150 m in the clearest oceanic water. The lower boundary is defined by the compensation light depth, where only enough light is present for photosynthesis to balance plant respiration over 24 hours. The disphotic zone is dimly lighted; there is sufficient light for vision, but too little for plant production. The deepest and largest zone is the aphotic zone, a region of darkness extending to the seafloor where the only light emanates from the bioluminescence of certain animals.
Infrared wavelengths are absorbed within the first few metres of the surface and are the primary heat source of the oceans. Sea surface temperatures vary with latitude and fluctuate seasonally but remain within a moderate range of about 40deg C to -1.9deg C, the freezing point of water with a salinity of 35.
In many parts of the ocean, there is thermal stratification consisting of an upper mixed layer of water of almost homogeneous temperature; a region of rapid temperature decrease known as the permanent thermocline; and an underlying cold deep layer of water formed originally at the surface in polar regions.
In mid-latitudes where seasons are pronounced, seasonal thermoclines are formed in the surface layer during spring and summer. These zones of steep temperature change are established because increased solar radiation elevates surface temperatures at a time when lessened winds reduce the amount of mixing in the water.
The average salinity of the open ocean is about 35 parts per thousand by weight, with ten major ions making up about 99.9% of all the dissolved substances in the oceans. In inshore or isolated areas with little wtaer exchange, salinity may vary from about 5 to 25 in brackish waters, to more than 40 in such hypersaline areas as the Red Sea and some shallow lagoons. Variations in salinity are primarily caused by evaporation 9which elevates salinity) and precipitation (which decreases salinity).
Whereas total salinity is variable, the major dissolved ions are not significantly affected by biological or chemical reactions and the relative proportions of these dissolved constituents remain constant.
The combined properties of salinity and temperature are used to define water masses. Each of these large bodies of water has a discrete origin and forms a distinctive environment, supporting a distinctive community of pelagic organisms.
Salinity, temperature and pressure establish the density of seawater. Changes at the sea surface that result in higher density will lead to downwelling of that water. Very dense water formed at high latitudes sinks to form the bottom water masses of the oceans, and this process is important in maintaining oxygen levels at all depths. Upwelling of water is partly caused by wind-driven mixing and is of importance in returning biologically essential elements to surface waters, where they are used by plants in photsynthesis.
The salt content of the sea lowers the temperature of maximum density and depresses the freezing point of seawater relative to freshwater. This not only results in the winter downwelling of polar water (see above), but it also prevents sea-ice formation except in polar areas and in shallow high-latitude marginal seas.
Oceanic surface currents are generated by global wind systems, and their direction is modified by the Earth's rotation. This results in large clockwise-moving gyres in the northern oceans and anticlockwise gyres in the Southern Hemisphere. The patterns of movement and mixing of these currents produces geographic regions of differing biological productivity. Horizontal transport of water also establishes the geographic distribution of many marine species.
Hydrostatic pressure effectively increases linearly with depth, at a rate of 0.1 atm me-1. In the deepest areas, organisms live at pressures exceeding 1000 atm.
The marine phytoplankton community is composed of several diverse groups of algae that carry out autotrophic production and begin the pelagic marine food chain. Photosynthesis results in the produciton of high-energy organic materials from carbon dioxide and water plus inorganic nutrients.
Photosynthesis involves a series of interrelated chemical reactions. The light reactions depend upon chlorophyss and accessory pigments capturing photons of light, so that radiant energy is converted to chemical energy. The dark reactions do not require light; they reduce the carbon dioxide and produce high-energy carbohydrates as end products. Respiration in plants and animals is the reverse process of photsynthesis, whereby oxygen is used to release the energy contained in carbohydrates and carbon dioxide is liberated.
All phytoplankton species require certain inorganic substances to carry out photosynthesis, including sources of nitrogen, phosphorus, and iron (also silica for diatoms) which may be in concentrations that are low enough to be limiting to plant production. Some species also require certain organic substances (eg vitamins) for auxotrophic growth, and these also may be present in limiting concentrations.
Estimates of the total phytoplankton crop (standing stock or biomass) in a particular locality can be determined by measurements of cell numbers, total volume, or most commonly, by quantity of chlorophyll a. The rate of primary production is most often measured by following the uptake of radioactive 14C in samples of seawater containing phytoplankton.
The amount of photosynthesis increases with light intensity up to a maximum value known as Pmax which is specific for each species. When light intensity increases beyond this value, the rate of photsynthesis declines due to photinhibition. The light intensity at which plant photosynthesis (production) exactly equals plant respiration (losses) is the compenstion intensity. Gross photosynthethis describes total photosynthesis; net photosynthesis is gross photosynthesis less respiratory losses.
Photosynthetic responses of phytoplankton species to light can be described by a series of equations based on values for Pmax and K1. Pmax values are generally higher at warmer temperatures and in eutrophic waters.
Phytoplankton are exposed to differing light intensities as light changes over the course of a day and as the algae are mixed vertically in the surface layers of the sea. At the critical depth, photosynthetic gains throughout the water column are just balanced by respiratory losses in the phytoplankton. If the depth of water mixing is greater than the critical depth, no net primary production can take place. Net production occurs only when the critical depth exceeds the depth of mixing.
Growth rates of phytoplankton are also controlled by the concentrations of essential nutrients in seawater. Oligotrophic regions have low concentrations of essential nutrients and therefore low productivity. Eutrophic waters contain hgih nutrients and support high numbers of phytoplankton.
Each species of phytoplankton has a particular response to different concentrations of limiting nutrients, and each has a different maximum growth rate. These differences and the species-specific responses to different light intensities, temperatures, salinities and other parameters, mean that heterogenous and fluctuating environmental conditions favour different species at different times and allow many species to coexist in the same body of water. Thus phyoplankton species diversity can be high in what appears superficially to be a homogeneous aqueous environment.
Solar radiation and essential nutrient availability are the dominant physical factors controlling phytoplankton production in the sea. The amount of light varies with latitude and the amount of nutrients contained in the euphotic zone is largely determined by physical factors controlling vertical mixing of water.
Despite year-round high light intensity, tropical regions are generally low in productivity because solar heating stabilizes the water column and nutrients remain at low concentrations within the euphotic zone. Conversely, polar regions are generally high in nutrients but low in solar radiation except for a brief period in the summer. Maximum annual productivities are generally found in temperate latitudes where light and nutrients are both reasonably abundant.
The general latitudinal patterns of primary productivity are altered by a number of different physical processes that lead to nutrients being redistributed in the water column in discrete areas. These processes occur on scales varying from very large (eg gyres and continental upwelling), to smaller (eg tidal fronts and rings), to the very small scales of Langmuir circulation in which only the top few metres of the water column are mixed.
The standing stock of phytoplankton in the surface layers of the sea ranges from less than 1 mg chlorophyll a m-3 to about 20 mg m-3 during a phytoplankton bloom. Regional oceanic primary productivity ranges from <50 to >600 g C m-2 year-1, with coastal upwelling regions having the highest values. Total primary productivity of the world ocean is about 40 x 109 tonnes of carbon per year, a figure that is approximately equivalent to terrestrial plant production.
Zooplankton grazing removes different proportions of the phytoplankton production in different marine areas. Much of the plant production is consumed in areas where growth rates and generation times of the zooplankton permit tight coupling with any phytoplankton increase (eg tropical waters). Where there is a lag in the development of zooplankton relative to increases in phytoplankton biomass, then some of the algal community dies and sinks to become a food source for deeper-living pelagic or benthic animals (eg North Atlantic).
The vertical profile of phytoplankton production changes with season and with latitude. High surface productivities generally occur in temperate latitudes in spring and autumn, whereas chlorophyll and productivity maxima occur considerably deeper in tropical waters.
The marine zooplankton community includes many different species of animals, ranging in size from microscopic protozoans to animals of several metres in deminsion. The holoplanktonic species spend their entire lives in the pelagic environment; meroplanktonic forms are temporary members of the plankton, and include the eggs and larval stages of many benthic invertebrates and fish.
Although zooplankton are routinely collected by towing fine-meshed nets through the water, not all species are representatively captured by this method. Some animals are too small to be retained in nets, others are capable of detecting and evading nets, and some species are too fragile to survive colleciton by nets and subsequent processing in chamical preservatives. Direct observations of zooplankton using scuba techniques, ROVs, or submersibles have greatly increasd our knowledge of fragile and/or fast-swimming species.
The presence of meroplanktonic larvae in the water is linked to the reproductive patterns of the adults. In tropical regions, meroplankton are present throughout the year. In higher latidudes, the larvae of benthic invertebrates and fish appear seasonallky because reproduction in the adults is linked to higher temperatures and elevated phytoplankton production.
The vertical gradients of temperature, light, primary production, pressure, and salinity create distinctive environments at different depths in the water column. These vertical zones (epi-, meso-, bathy-, and abyssopleagic) are somewhat arbitrary in nature, bu tdifferent species of zooplankton generally inhabit discrete depth zones within the ocean. The life styles, morphology and behaviour of organisms living deeper in the water column differ from those exhibited by epipelagic species, and the biomass of zooplankton decreases exponentially with depth.
As light from the Sun diminishes with depth, bioluminiscent light produced by organisms becomes increasingly important as a means of communication. Many different species display the ability to produce light, and the biological significance of bioluminscence varies with the species. Some use light displays to attract potential prey, others to deter predators, some may use bioluminscence to attract mates or to form reproductive swarms.
Although most zooplankton have preferred depth ranges, many species move vertically in the water column with a diel periodicity. The most usual pattern is a nocturnal migration in which animals make a single descent to deeper water at sunrise. The adaptive significance of diel vertical migration may be different for different species. This bahaviour may allow animals to cnserve energy by remaining in colder waters except when feeding; it may reduce mortaility from visual predators; or it may permit animals of limited swimming ability to sample new feeding areas with each ascent.
Diel vertical migration has several important biological and ecological consequences. It probably enhances genetic exchange by mixing the membres of a given population; this results because vertical migrations are never precisely synchronized among all the members of a population. Some individuals begin migrations sooner or later than others, with the result that some members will eventually be lost form the original group and new members will be added. Secondly, diel vertical migrations increase the speed at which organic materials produced in the euphotic zone are transferred to deeper areas.
In high latitudes, extensive vertical migrations may be undertaken on a seasonal basis, and these are generally linked with reproductive cycles and development of larval stages. In such migrations, the adults are usually found in deeper waters during the winter when food is scarce; the developing young are present in surface waters during the spring and summer when phytoplankton is plentiful.
By moving vertically in the water column, zooplankton enter currents that are moving in different directions and at different speeds. Thus diel or seasonal vertical migrations that are attuned to particular current regimes can result in the retention of populations within favourable localities.
Present-day distributions of zooplankton have been established over geological time and reflect past dispersal patterns as well as the physiological and ecological requirements of the species.
pipelagic zooplankton are often associated with specific water mass types, which are established by latitudinal gradients in temperature, salinity, and other physico-chemical factors. Mesopelagic and bathypelagic species tend to have wider geographic distributions, reflecting increasing homogeneity in environmental conditions with increasing depth.
The numbers of species of epipelagic and mesopelagic zooplankton are higher in low latitudes, but the numbers of individuals tend to be relatively low. The reverse situation is found in high latitudes, where there are fewer species but with higher abundance.
Within the bounds of their geographic regions, zooplankton exhibit patchy distributions on a wide range of pace- and time-scales. Patchiness may result from responses to physical turbulence or mixing, or to chemical gradients such as salinity changes. Patchiness may also result from interactions between prey and predators, or it may reflect other biological events such as reproduction.
Long-term records indicate that plankton abundance and species composition may change substantially over decadal time scales. Decreasing plankton biomass may be caused by climate changes that increase water stratificiation and depress upwelling; conversely, in other regions, increasing winds may enhance nutrient concentraations in the euphotic zone and lead to increaed phytoplankton and zooplankton production.
Food chains are ways of describing the linear passage of energy and organic materials contained in food from the first trophic level of primary producers, through the consumer levels of herbivores and carnivores, to the top-level predators. There is an energy loss with each transfer between trophic levels because of metabolic demands and conversion of chemical energy to heat. However, chemical elements that are incorporated in food are recycled through the decomposition of organic materials; this process releases dissolved inorganic compounds that can once again be taken up by phyoplankton and converted to organic compounds during photosynthesis.
Despite great differences in size between phytoplankton and consumers in higher trophic levels, the differences in generation times (hours to many years) among these organisms result in very similar biomass values in each trophic level of marine food chains.
Estimates of secondary production in different marine localities can be made by using the expression P(n+1) = P1En. This equation combines quantitative values for primary productivity in an area with the number of trophic levels in the food chain, and with the ecological efficiency at which energy is tranferred from one trophic level to another.
The number of trophic levels in a food chain is inversely correlated with the predominant size of the phytoplankton. Food chains in nutrient-rich upwelling areas are characterized by having large chain-forming diatoms, high primary productivity, few trophic levels and a high biomass of fish or marine mammals. In the nutrient-poor open ocean, the primary producers are nanoplanktonic autotrophic flagellates with relatively low productivity; this leads to long food chains and, because of increased energy loss in longre food chains, there is a relatively lower biomass of top-level predators.
Food webs are more realistic, but more complex, depictions of energy flow through interacting species. They are a means of recognizing that many marine species compete for the same food items, that many animals change diets during life, that some organisms feed primarily on detritus, and that cannibalism is common in the sea. Such relationships may affect the amount of energy that is available for top-level predators; for example, competition for food between ctenophores and larval fish, or ctenophore predation on fish eggs and fish larvae, may significantly lower fish stocks that are harvested commercially.
Bacteria and planktonic protozoans interact in a microbial loop that is coupled with the classic phytoplankton-zooplankton-fish food chain. In this subsystem, bacteria decompose particulate and dissolved detritus; the resulting bacterial production is consumed by protozoans and by some larger zooplankton, such as invertebrate larvae and appendicularians. Thus bacteria regenerate dissolved nutrients for subsequent utilization by phytoplankton, they themselves form a source of food for planktonic bactivorous specis, and the bacterial production is transferred to higher trophic levels by the intermediary links of protozoans which are fed on by larger plankton.
Major difference exist between marine and terrestrial food webs. The majority of marine primary production is carried out by fast-growing microscopic phytoplankton, most of which is consumed and assimilated by herbivores. In contrast, most terrestrial vegetation is large, slower growing, and contains much indigestible structural material. Only 15% or less of the total terrestrial plant production is eaten, and only a fraction of this is digestible and assimilated into herbivore production. Further, the dominant marine animals are poikilothermic, with lower metabolic energy demands than the homoiothermic birds and mammals that live in terrestrial habitats. This difference in energy utilization, coupled with the fact that most marine primary production is eaten, means that energy is transferred with greater efficiency through marine food chains, and htat there is a much higher secondary production in the sea compared with that on land.
Whereas it is possible to obtain fairly accurate estimates of primary production through various techniques, and fish catch statistics provide minimum values for energy output from marine food webs, it is much more difficult to quantify secondary production in the intermediate trophic levels occupied by zooplankton and smaller nekton. Although some techniques have been applied to measure secondary production from field data, these are often impracticable bcause of the vast geographic areas under consideration and the continual movement of the water and resident organisms. Many researchers have therefore resorted to different experimental options. These include laboratory-scale experiemnts, which attempt to quantify each aspect of energy partitioning in a species; the use of controlled ecosystem experiments, which are carried out on a larger scale and attempt to study several interacting trophic levels at one time; and computer model simulations, in which data from various sources are entered into mathematical models that attempt to simulate natural processes.
The term 'mineralization' describes the process whereby elements that have passed through food webs are recycled. Ecological studies are particularly concerned with the recycling rates of essential nutrients that may be present in limiting concentrations in the sea; these include nitrate, iron, phosphate and, occasionally, dissolved silicon. Of these nutrients, nitrate is the one that is most often present in sufficiently low concentrations to limit plant growth. Nitrogen has a complex cycle in the sea because it occurs in many forms; nitrate is the dominant form most often utilized by phytoplankton, but ammonia, nitrite, and dissolved molecular nitrogen can also be used by some species. The physiological activities of organisms produce particulate and dissolved organic nitrogen in various chemical species, and different types of bacterial mediate the conversions from one type of nitrogen compound to another.
An important distinction is made between regenerated nitrogen (primarily ammonia and urea) that is recycled in the euphotic zone by pelagic organisms, and new nitrogen (primarily nitrate) that enters the euphotic zone from upward movement of deep water or, in smaller amounts, from river inflow and precipitation. The amount of new nitrogen relative to regenerated nitrogen (the f-ratio) is high in upwelling regions and low in oligotrophic areas. Where production is based primarily on new nitrogen (nitrate), teh photosynthetic quotient (PQ is high, indicating the formation of proteins from nitrate with release of oxygen. If photosynthetic production is based on regenerated nitrogen forms (ie ammonia), the PQ is low as oxygen is required in the reaction. It is the continual input of new nitrogen that can elevate primary production levels and ultimately sustainable fish harvests.
Carbon is essential for life, and is never present in limiting quantities in the sea. This is because dissolved carbon dioxide enters into equilibrium reactions with bicarbonate and carbonate ions. As more CO2 enters the sea from the atmosphere, or as the result of physiological activities (primarily respiration), more bicarbonate and carbonate ions are formed, thus increasing the amount of CO2 which can continue to enter the sea. Conversely, when there is a biological demand for dissolved CO2, the chemical reactions are reversed and CO2 is released from its bound ionic states. As the quantities of CO2 entering the atmosphere from human activities increase, it becomes increasingly important to determine how much can be absorbed by the seas and how much will accumulate in the atmosphere where it may contribute to global warming.
The nekton comprises the larger, pelagic, marine animals whose swimming abilities are such that their movements are independent of ocean currents. Included in this category are larger crustaceans (some euphausiids, shrimp and swimming crabs), squid, sea snakes, marine turtles, and marine mammals, with adult fish making up the dominant fraction. Seabirds are also considered here because they are dependent on the sea for food and may have considerable influence on the neuston and epiplanktonic communities.
Few commercially harvested crustaceans are pelagic, but some of the larger, very abundant euphausiid species are presently fished in the Antarctic and off Japan. The superabundance of the Antarctic krill (Euphausia superba) makes it an attractive fishery target, and it is likely that this harvest will increae, despite high economic costs.
Squid form another abundant invertebrate group targeted by fisheries. More needs to be known about the biology and abundance of these animals before fisheries management can be effective in protecting these stocks. It has become evident that driftnet fishing for squid is very unselective, and that vast numbers of seabirds, fish, turtles, and marine mammals have been inadvertently captured and killed by this method. More selective, alternative squid-fishing techniques are available, although their use increases the costs of fishing.
Eight species of turtles, one lizard, and about 60 species of snakes are the only reptiles to have become marine. The turtles have become endangered species from hunting of the adults and their eggs.
There are about 110 species of marine mammals. The largest are the baleen whales that feed by filtering zooplankton or fish (or benthic invertebrates, in the case of the grey whale) through their plates of baleen. The toothed whales (including dolphins and porpoises) are predators in the sea. These to groups of cetaceans together consume a much greater quantity of marine biomass than is removed from the oceans by the entire commercial fishery.
Many species of whales, pinnipeds (seals, sea lions and walruses) and sirenians (manatees and dugongs) have been extensively hunted. Their low fecundity and long development times from birth to maturity make them especially vulnerable to rapid depletion of population numbers through commercial harvests. Many of the species are now endangered, and recovery of populations is slow.
The most highly adapted of the seabirds spend 50-90% of their lives at sea, but all remain dependent upon land for nesting sites. The highest numbers are found in association with very productive waters, where zooplankton and fish are concentrated. Non-migrating species are subject to natural mortalities caused by climate change and subsequent declines in their prey (eg the effects of El Nino on the guano birds of Peru). Human-induced mortalities such as overfishing, habitat destruction, introduciton of predators and coastal pollution increasingly threaten seabirds.
The great majority of marine fishes are teleosts with a bony skeleton, and the 20 000 or so species show considerable diversity in terms of anatomy, behaviour and ecology. The most abundant species are epipelagic plankton-feeders with very high fecundity, and many of them (eg herring, sardines, anchovies) form the basis for some of the most profitable marine fisheries.
There are fewer deep-water species of fish, and they are not as numerous as their sahllower-living relatives. Many of the meso- and bathypelagic fishes tend to be relatively small. Many of the species have photphores and use bioluminescence to locate or lure either prey or mates, or to evade predators.
In 1993, the total world catch of all marine species of fish (including squid and shellfish) was about 84 million tonnes per year. About 64% of this catch was taken from the largest of the oceans, the Pacific Ocean, with China, Peru and Japan being the leading fishing nations.
Although various attempts have been made in the last 100 years to manage fisheries, these have met with little success. Many fish stocks or fishing regions have been depleted or are in danger of being overfished.
Fisheries management has traditionally been based largely on stock/recruitment theories that have ignored the role of the environment in causing natural fluctuations in the numbers of fish. It is becoming increasingly apparent that the recruitment of larval fish into adult stocks can vary greately depending on whether there is sufficient food for young fish, whetehr predation is high or low, whether larval fish are transported by currents into unfavourable habitats, whether disease affects the population, and whether growth is slowed or hastened by temperature, food availability, or other factors.
Biological oceanography can assist fisheries in two ways. The first way is by increasing our understanding of what factors cause natural fluctuations in the abundance of fish. The second way is in providing near real-time oceanographic data in connection with the actual process of fishing; better information on the location of fish schools can reduce the cost of the fisheries.
Mariculture is another way of increasing the yield of fisheries resources. At present, only a few o fthe many marine species are under culture, but cultivation is expected to expand from its present production of about 5 x 106 tonnes per year.
The benthic environment is divided into a number of distinctive ecological zones based on depth, seafloor topography, and vertical gradients of physical parameters. These are the supralittoral, littoral, sublittoral, bathyal, abyssal, and hadal zones.
Benthic plants include macrophytic angio-sperms like mangrove trees, marshgrasses, and seagrasses. Macrophytic algae include green, red and brown seaweekds, and the long-stemmed kelps, a type of brown algae. Microphytic algae include benthic species of diatoms. Cyanobacteria and dinoflagellates.
The number of phyla and the number of species of benthic animals exceeds those of pelagic species, at least partly because of the greater physical variety of benthic habitats.
Benthic animals are separated into infaunal and epifaunal species, depending upon whether they live within sediments or on the surface of the seafloor, respectively. Size categories of the zoobenthos consists of the large macrofauna (>1.0 mm), the small meiofauna which is characteristically found in sand and mud, and the microfauna which is made up mostly of protozoans.
Benthic primary productivity is measured by a variety of methods including the carbon-14 method for microphytic species, and harvesting and weight measurements of macrophytic plants. Methods of estimating benthic secondary production are similar to those employed for pelagic animals and described in Chapter 5.
The numbers and types of species making up any particular benthic community are determined by a variety of physical and biological factors. In shallow coastal communities, the types of species present and their relative abundance will be partly determined by tidal levels and degree of exposure to air, wave action, and range of salinity and temperature. At all depths, the type of sediment (eg sand, rock, mud) will dictate the relative proportions of epifauna and infauna. Biological factors that influence benthic community structure include competition for limited resources (eg food, space), predation, and type of development.
Benthic animals may have direct development, in which there is no free-swimming larval stage, or they may produce pelagic planktotrophic or lecithotrophic larvae. Planktotrophic larvae are relatively small and are produced in large numbers; they must fee don plaktonic food, and they remain in the water column for several weeks or months. Although they are a means of dispersal for the species, planktotrophic larvae have high rates of mortality, and the adult populations have variable rates of recruitment from year to year. Lecithotrophic larvae hatch from relatively large eggs that contain large amounts of nutritive material and that are produced in small numbers; these larvae do not remain planktonic for long, and they do not feed while in the water column. Compared with planktotrophic larvae, mortality rates are lower for lecithotrophic larvae, and species with this type of development tend to have low but constant biomass because recruitment is less variable.
Relative to most other marine habitats, intertidal areas are characterized by great fluctuations in environmental conditions. Littoral plants and animals are specially adapted to cope with variable temperatures and salinity, and to withstand periodic exposure to air.
Rocky intertidal regions support dense comminities with a high proportion of epiflora and epifauna that may compete for limited space. Many of the sessile species are arranged in distinct vertical zones. The upper boundary of any particular zone is often set by physiological limits of the species, such as tolerance to desiccation and temperature change. The lower limits of zones are generally established by biological factors such as predation and competition.
Intertidal areas of sand beaches support communities in which the primary producers are benthic species of diatoms, dinoflagellates, and blue-green bacterial, and the resident animals are predominantly infauna and meiofauna. The femiofauna are specially adapted to live on sand grains, or in the interstitial spaces between the particles, by their small sizes, elongate shapes, protected integuments and adhesive organs.
Annual primary production averages about 100 g C m-2 in rocky intertidal areas, with a maximum of 1000 g C m-2 in particularly favourable areas. Benthic primary productivity in sand beaches is less than 15 g C m-2 yr-1, and this system relies on energy derived from detritus and from primary production in the surrounding water.
Subtidal kelp forests occur on rocky substrates in cold temperate regions. Kelp are among the fastest growing of any plants, and the productivity of kelp beds ranges from about 600 to more than 3000 g C m-2 yr-1. Much of this production is not consumed directly, but enters the detritus food chain. Sea urchins are dominant components of kelp communities, and their feeding activities greatly influence the community structure. In some North Pacific kelp beds, otters are teh top predators that act as keystone species.
The circulation pattern of estuaries results in entrainment of nutrients and makes them some of the most productive of marine ecosystems. The upper reaches of estuaries are occupied by saltmarsh communities with total annual primary production generally ranging between 300 adn >3000 g C m-2. Seagrass beds typically form in intertidal areas of the middle reaches of estuaries, and total primary production by the seagrasses and associated epiphytes is about 600 to 1000 g C m-2yr-1. Both communities are dominated by detritus-based food chains. Estuaries also have subtidal mudflats or subtidal sand banks in which annual primary production (mostly by epipsammic algae) ranges from 10 to >200 g C m-2.
The high productivity of estuaries supports dense populations of animals in some areas. However, many animals are excluded from living in estuaries because of the fluctuating salinity, and thus species diversity is low.
Coral reefs are formed by stony corals that contain symbiotic dinoflagellates called zooxanthellae. The algae utilize carbon dioxide and waste products of the coral in photsynthesis, and in return the coral is provided with organic compounds such as glucose and glycerol. Photosynthetic fixation by the zooxanthellae provides only part of the energy required by corals; the remainder is supplied by predation on zooplankton and bacteria, and by absorption of dissolvewd organic matter.
Primary producers of coral reefs include phytoplankton, benthic algae, and zooxanthellae. Gross primary productivity is very high, ranging from about 1500 to 5000 g C m-2yr-1, but production to respiration ratios usually are between 1.0 and 2.5, and very little new nutrient material enters the system (ie f-ratio is <0.1(> put that in there to balance it!)). The high production of this system supports a community with very high species diversity.
Corals grow relatively slowly, at rates of less than 1 to 10 cm yr-1. Growth of a reef is also controlled by bioerosion and physical events (eg storms) that destroy the carbonate framework. Net vertical upward growth of reefs varies from a few to almost 30 mm yr-1 under favourable conditions.
Mangrove swamps occur along 60-75% of tropical and subtropical coasts. The major primary producers of these communities are salt-tolerant terrestrial plants that can live in oxygen-poor muddy substrates. The roots of the mangroves provide attachment sites for epifauna, and leaf fall is a major source of nutrients and energy for the detritus-based food chain. Net primary production is estimated to be between 350 and 500 g C m-2 yr-1.
The bathtal and abyssal zones together constitute over 90% of the benthic environment. Deposit-feedinb infauna predominate in organically-rich sediments, and the meiobenthos are particularly diverse. Benthic biomass diminishes rapidly with increasing depth, and annual secondary production is between 0.005 and 0.05 g C m-1. Most areas of the deep sea depend upon the fall-out from production in the euphotic zone, but only a small proportion of sinking organic matter reaches the seafloor in depths over 2000 m, and food limitation greately influences biological processes and community structure of the deep-sea benthos. In general, typical deep-sea inhabitants exhibity slow metabolic rates, slow growth rates and low fecundity.
Ocean trenches characteristically have a high proportion of endemic species. The biomass of hadal animals ranges from about 0.008 g m-1 in trenches far from land and underlying oligotrophic water, to as much as 9 g m-2 in trenches that lie near land under eutrophic water.
Hydrothermal vents and cold seeps support unique communities that are independent of solar energy and photsynthesis. Instead, the food chain in these environments is based on the presence of hydrogen sulphide that is utilized by chemosynthetic bacteria to form organic compounds from carbon dioxide. The bacteria are the primary producers in these communities, and they are either consumed directly by animals or they are found in symbiotic relationships with animals.
Deep-sea vents and seeps support extremely dense concentrations of large animals, and biomass may be as much as 30 kg m-2. Although these environments have plentiful food and, in the case of vents, temperatures that are higher than usual in deep water, relatively few animals have developed the ability to live in high concentrations of H2S and species diversity is low.
The annual fish harvest of more that 100 x 106 tonnes has had a greater impact on the ocean than any other human activity. By 1995, fish catches were declining in all major oceans except the Indian Ocean and 70% of the ocean's fish stocks were either being fully exploited, were over fished, or were recovering from being overfished.
The results of intensive fishing include: declines in targeted fish stocks and consequent changes in relative abundance of species; changes in size structure of fish populations; declines in pelagic and benthic animals captured incidentally as by-catch; acceleration of nutrient transfer to deep water through dumping of deat by-catch; increased numbers of scavengers in marine food chains receiving large amounts of by-catch' and destruction of seabed habitats through benthic trawling.
Fish stocks are also affected by construction of dams that eliminate spawning grounds (eg salmonids) and by loss of coastal spawning and nursery grounds due to land reclamation or pollution.
Human activities result in the release of a variety of pollutants into the sea. These substances, which may cause deleterious changes, include petroleum hydrocarbons, plastics, pesticides and related chlorinated hydrocarbons, metals, fertilizers, and radioactive wastes. Sewage outfalls deliver many of these pollutants as well as human wastes, detergents, and pathogenic bacteria and viruses. Power plants also release heated effluents that elevate ambient seawater temperatures.
Although oil spills are among the most visible types of marine pollution and ecological damage in the immediate site may be severe, populations generally recover within 5 to 10 years. In the open ocean, the accumulated concentration of petroleum hydrocarbons is too low to cause measurable effects.
Nonbiodegradable plastic materials are now found throughout the oceans. These include lost fishing nets, which may continue to entangle animals for years, and plastic materials that are mistaken for prey and ingested by turtles and seabirds.
Toxic synthetic pesticides like DDT and related compounds (dioxins, PCBs) that enter the marine environment are not readily degraded; they persist for long periods and enter marine food chains. Because they are stored in fat tissues, these compounds show biomagnification, with higher trophic level animals accumulating concentrations that may be lethal. Past incidences of pesticide and PCB poisoning in marine organisms have led to bans on usage and production in some countries.
Accumulations of heavy metals (eg mercury, copper) resulting from industrial outfalls may cause serious human health problems. Historically, the damaging effects caused by humans eting mercury-contaminated seafood were shown in the 1950s in Minamata, Japan, where more than 2000 people were directly affected. Now regulations and monitoring programmes exist to limit and detect unacceptable concentrations of these metals in marine products.
Sewage disposal is a major form of coastal pollution throughout the world. Nutrients in human wastes and those in detergents and fertilizers enrich local waters. This eutrophiciation may be beneficial in some cases, but often the amount of nutrients delivered leads to excessive plankton blooms that eventually decay and cause oxygen depletion. Pathogenic organisms in human wastes, like the cholera virus, can be filtered out of water near sewage outfalls by mussels and clams, and then be transmitted to humans who consume this seafood.
Radioactive waste do not presently occur in concentrations that threaten marine life, although it is known that some organisms, particularly seaweeds and bivalves, can accumulate radionuclides from waters around nuclear plants.
Power plants release heated water that elevates ambient seawater temperature and thereby affects marine communities within the immediate area. In some cases teh heated effluent is used to enhance growth rates of organisms grown in culture, but often the community changes are detrimental or unwanted.
Some marine communities and ecosystems have been changed through the deliberate or accidental transplantation of species. Increased commercial shipping has accelerated the rate of introduction of species into new environments, with an estimated 3000 species of marine plants and animals being carried daily across oceans in the ballast tranks of ships. Many do not survive but some, like the ctenophore Mnemiospis or the zebra mussel, have major impacts on their new environments.
Estuaries and mangrove swamps are productive coastal ecosystems that constitute important spawning and nursery grounds for many fish, harbour shellfish populations and provide rich feeding grounds for birds. As well, mangrove swamps buffer coastlines from erosion and inundation during tropical storms. However, these ecosystems are often heavily affected by hman activities such as land reclamation, disposal of sewage and industrial wastes, and eutrophication.
Coral reefs are declining throughout the world. Expanding human populations near reefs and increasing tourism have accelerated development and have brought growing pressure to exploit reef resources. Coral reefs are detrimentally affected by increased sedimentation resulting from land development and subsequent erosion, and from eutrophiciation stemming from sewage disposal and agricultural runoff. In some locales, the coral is mined as building material. Destructive fishing techniques remove large numbers of fish, change species composition on the reef, and damage corals directly. Corals world-wide have been affected by elevations in seawater temperature, and global warming is a potential danger to reef communities.