- Email: [email protected]
Ocean Ecosystems
O Ocean Ecosystems Richard T Barber, Duke University, NC, USA r 2001 Elsevier Inc. All rights reserved. This article is reproduced from the previous edition, volume 4, pp 427–437, r 2001, Elsevier Inc.
Glossary Convective mixing Vertical mixing produced by the increasing density of a fluid in the upper layer, especially during winter in temperate and polar regions. Euphotic zone The surface layer of the ocean in which there is adequate light for net positive photosynthesis. Nutrients Dissolved mineral salts necessary for primary productivity and phytoplankton growth: Macronutrients are phosphate, nitrate, and silicate; micronutrients are iron, zinc, manganese, and other trace metals. Phytoplankton Photosynthetic, usually single-celled, plants that drift with ocean currents. Pycnocline The layer in which density changes most rapidly with depth and separates the surface mixed layer from the deep ocean waters. Southern Ocean The circumpolar ocean in the Southern Hemisphere between the Subtropical Front and the continent of Antarctica.
History of the Ecosystem Concept Natural scientists have long recognized that beyond populations or communities there is a higher level of organization, but it has been difficult for physical and biological scientists to reach consensus about its nature. The origins of the ecosystem concept are indistinct and, indeed, involved polemics between some of the principals. Ernst Haeckel (1838–1919) coined the word ‘‘ecology’’ and he envisioned ecology as a science that studies the environment as a stage within which selection pressures and adaptations affect the evolution of species. Haeckel brought the physical environment to the forefront, but he was interested primarily in evolution, which acts on species or genetically interacting populations. Victor Hensen (1835–1924) coined the word ‘‘plankton’’ to describe the organisms that drift in the ocean following the paths dictated by currents and mixing. Hensen was interested in the integrated working of natural systems, the subject today called ‘‘ecosystem science.’’ He was particularly interested in the functional behavior of ocean systems from the smallest phytoplankton up
Encyclopedia of Biodiversity, Volume 5
Stratification The formation of distinct layers with different densities; stratification inhibits mixing. Subpolar Pertaining to the regions between the polar and temperate zones, but for the oceans the boundaries are the Subtropical Front and the Polar Front. Subtropical Pertaining to the regions which, under the influence of the trade winds, are permanently stratified. Upwelling Upward vertical movement of water through the bottom of the surface mixed layer produced by a divergence at the surface. Zooplankton Animals that float or drift with ocean currents: Microzooplankton are protozoan plankton that graze on small phytoplankton; mesozooplankton are crustaceans that graze on larger phytoplankton such as diatoms.
through the food web and leading finally to fish, birds, and marine mammals. Hensen and Haeckel had a long professional dispute which seems ironic today because the eventual synthesis of their ideas by others led indirectly to the ecosystem concept. The linguistic route to the modern word ‘‘ecosystem’’ was tortuous. From 1877 to 1939 the words ‘‘biocoenose,’’ ‘‘microcosm,’’ ‘‘naturkomplex,’’ ‘‘holocen,’’ and ‘‘biosystem’’ were all proposed as names for this idea. In 1935 an English scientist, Tansley, introduced the neologism ‘‘ecosystem’’ and this term rapidly eclipsed competing names, but ecosystem science was not a well-organized endeavor until 1959, when Eugene Odum and Howard Odum published a groundbreaking textbook, Fundamentals of Ecology. There were many treatises on ecology before the Odums’ book, but their book stressed the principle that the ecosystem (not the population or community) is the fundamental unit of ecology and indeed of biology. The 1959 text, which develops this concept in each of its chapters, has remained in print for many years. Although Eugene Odum’s definition of ecosystem (given at the beginning of this article) is widely accepted, the principle of the ecosystem as the
http://dx.doi.org/10.1016/B978-0-12-384719-5.00188-X
581
582
Ocean Ecosystems
fundamental unit of ecology is controversial. Populations, communities, adaptation, and evolution have all been explosively successful research areas. Ecosystem research, because of its inherent complexity and requirement for interdisciplinary work, has not enjoyed the success of the more reductionist areas of ecological research. The study of ecosystems is usually supported by encouraging words, but our institutions and agencies, which are organized along disciplinary lines, have trouble coping with a science composed of equal parts of physics, chemistry, and biology. Ecosystem studies are placed with the life sciences, in which there is little institutional or agency enthusiasm for the complex and expensive atmospheric and physical oceanographic work required by the study of ocean ecosystems.
Utility of the Concept The ecosystem concept was originally developed for terrestrial, intertidal, and benthic habitats, in which ecological succession on timescales of 10–100 years is an obvious and salient characteristic of the ecosystem. Succession on this timescale is difficult or impossible to detect in the fluid medium of pelagic ocean ecosystems. In the ocean, winds, mixing, and currents appear to reset the successional time clock to time zero each annual cycle or, perhaps, with each storm or passage of a front or large eddy. It has been shown that there are low-frequency (over the timescale of 10–100 years) changes in species abundance and community structure. These biological changes are sometimes so far reaching that they are called ‘‘regime shifts,’’ the term used by John Steele in 1998, but they are not functionally analogous to succession, which is (i) orderly and directional, (ii) involving biological modification of the physical environment, and (iii) resulting in a more stable climax community (Odum, 1969). Succession in ocean ecosystems is dramatically evident on the day to month timescale after spring stratification in temperate and subpolar waters or after an episode of upwelling. This short timescale succession, which appears to be cyclic, is not unidirectional and definitely not orderly. If long-term directional succession does not appear to occur in pelagic ocean ecosystems and succession is a central tenet of ecosystem theory, isn’t the very existence of ocean ecosystems in doubt? The answer is that ocean ecosystems, as Steele (1985) noted, are clearly different from terrestrial, aquatic, intertidal, or benthic ecosystems, but ocean ecosystems meet most of the requirements of the definition set forth by Eugene Odum. The ocean ecosystems described here all have a characteristic and distinct trophic structure, characteristic and distinct material cycles, some degree of internal homogeneity and commonality, and definable hydrographic boundaries. There is considerable heuristic power in the ecosystem concept because understanding gained in one ocean ecosystem can be used to predict the response of another ecosystem of the same kind that is geographically distinct from it. This predictive power is perhaps the greatest benefit of ecosystem theory and provides evidence that each distinct kind of ocean ecosystem has characteristics that can be generalized and used in prediction. Pomeroy and Alberts (1988) emphasized that the concept involves emergence of new properties. One consequence of the hierarchic organization of ecosystems is that as
components (both biotic and abiotic) are integrated into a larger functional unit, new properties emerge that cannot be detected by study of the component populations or processes, no matter how thorough the reductionist study. This aspect of ecosystem theory makes the concept useful, even necessary, for predictive understanding of ocean ecosystems. Determining specific emergent properties of a system as large as ocean ecosystems is difficult. Ocean ecosystem spatial domains of thousands of kilometers are difficult to sample adequately with ships; however, with satellites that measure wind, ocean temperature, ocean currents (from sea surface topography), and phytoplankton biomass, a new era has begun in understanding ocean ecosystems. Are there appropriate benefits to justify the large societal investment in remote sensing and data handling required to achieve this new level of understanding? One benefit deals with fisheries management. The historic approach to management of these fisheries has involved analysis of local populations and environment. It has become clear that ecosystem properties, especially physical conditions, operating on scales much larger than the range of the exploited population, may be responsible for changes in reproductive success and adult abundance. In this context, a valuable societal payoff of understanding ocean ecosystems is improved ability to predict local variations in living resources as shown by Sherman and others in 1986. Odum repeatedly emphasized that ecosystem science and economics are parallel disciplines and expressed regret that they are not perceived as such, particularly by the economic community. Investors and political leaders making policy decisions on resource development need an understanding of the probability, frequency, and intensity of natural variability for realistic economic decision making. In this context, economics and the ecosystem concept are closely related: By understanding the variability of ocean ecosystems, decision makers will also understand that variation in marine resources is a normal and inevitable characteristic that must be accommodated in economic plans. This kind of resource-related benefit, although important, is not the only societal benefit that will accrue. Odum (1977) said, ‘‘It is the properties of the large-scale integrated systems that hold solutions to most of the longrange problems of society.’’ Although few in the scientific community recognized the wisdom of this comment two decades ago, the validity of Odum’s prediction has now been well demonstrated. For example, carbon dioxide modification of the planet’s heat budget, and therefore climate, is a phenomenon that can be understood only if the emergent properties of large-scale biogeochemical systems are understood.
Partitioning the Ocean into Natural Functioning Units Central Problem The central problem for the lower trophic level of ocean ecosystems is obtaining light (energy) and inorganic nutrients (mass). Odum’s definition requires that for a functioning system to be a distinct ecosystem it must possess characteristic trophic structure and material cycles. That is, how one kind of ocean ecosystem captures light and passes that energy on in
Ocean Ecosystems
583
the form of primary productivity, secondary productivity, and so forth is different from how another kind of ocean ecosystem processes and transfers its energy. Likewise, how mass (C, N, P, and Si), initially in the form of inorganic compounds, is taken up and transferred through the food web and eventually released back to the environment is different and very poorly understood. What controls the supply of light and nutrients to an ocean ecosystem? Sverdrup in 1955 was the first oceanographer to note that the spatial and temporal patterns of physical processes, particularly the seasonal patterns of mixing, stratification, and upwelling as well as the seasonal pattern of irradiance, control the patterns of biological organization. The division of ocean ecosystems into six distinct types is based fundamentally on the pioneering work of Sverdrup and that of others in the intervening years.
Using these criteria, Longhurst partitioned the ocean into four major biomes: the westerlies biome, in which convective mixing and stratification are forced largely by the strong seasonal progression of winds, irradiance, and heat flux; the trades biome, in which upwelling and mixing are forced on the oceanbasin scale by both local and remote wind forcing; the polar biome, in which there is no significant thermal stratification and mixed layer depth is constrained by a surface low-salinity layer which forms each summer as the marginal ice melts; and the coastal biome, in which coastal processes such as tides or currents break down stratification and force mixing. Longhurst’s (1998) partitioning of the ocean into four biomes and 55 provinces is a significant accomplishment. Eventually, his ideas will be merged with the ecosystem concept to produce an internally consistent hierarchy of biomes, ecosystems, and provinces.
Biome Concept
Ecosystem Concept
Longhurst (1998) presented a scholarly discussion of the attempt to partition the oceans into natural functional provinces. He described with considerable elan a regional ecology of the oceans which is clearly distinct from partitioning the ocean into ecosystems. The difference can be illustrated using one of the best described ocean ecosystems, the coastal upwelling ecosystem. It is well accepted that there is one coastal upwelling ecosystem and that it is replicated at five coastal regions in the world ocean: the California Current off California and Mexico, the Peru Current off Peru and Chile, the Benguela Current, the Canary Current, and the Arabian Sea off Oman and Yemen. Longhurst recognized the same regions but counted them as five distinct ocean ‘‘provinces.’’ The ecosystem concept emphasizes that there are geographic ocean regions that have much in common; ecological geography emphasizes the discreteness or autonomy of various ocean provinces. Both concepts are useful. Longhurst (1998) proposed that the following information is required to define the functional nature of an ocean region:
Using the formal Odum definition of ecosystem, six more or less well-defined ocean ecosystems can be delineated (Table 1), but note that the boundaries of these ecosystems are usually oceanographic, not geographic, features. (See Tomczak and Godfrey (1994) for a description of these oceanographic features). Although it is possible to describe approximately where these ecosystems exist, the actual domain is determined by dynamic processes such as fronts where two kinds of ocean water converge. The approach is to define the characteristic trophic structure and material cycles by applying the same analysis to each system. Each of the six systems has a different combination of stratification, nutrient supply (Table 1), primary productivity (Table 2), and biotic characteristics (Table 3). This analysis indicates that there are six distinct ocean systems that meet Odum’s criteria:
Latitude, which determines if wind stress induces mixing or wind-driven advection Depth of water because stratification may be broken down in shallow seas by tidal mixing Proximity to coastline, which determines the effects of terrestrial runoff, river discharge, and release of nutrients (especially Fe) from sediments Seasonal irradiance, which forces photosynthesis, stratification, and freezing or melting of ice Winds, which force mixing or upwelling of subsurface waters and their nutrients up to the euphotic zone Precipitation, which may induce strong stratification by making the surface layer less salty Nutricline depth, which modifies the vertical flux of nutrients by wind mixing and upwelling Strength of the vertical nutrient gradient, which determines the magnitude of the upward nutrient flux External source of iron, because insufficient iron may limit uptake of macronutrients, phytoplankton growth, or primary productivity
1. A low-latitude gyre ecosystem is present in each of the five great low-latitude gyresFNorth Atlantic, South Atlantic, North Pacific, South Pacific, and South Indian OceansFas well as in the warm pool of the western Pacific Ocean, the equatorial Indian Ocean, and in large marginal seas such as the Mediterranean Sea and the Gulf of Mexico. This ecosystem is also present in the Western Boundary Current regions between the western edge of each of the five great gyres and the coastal waters of the adjacent continent. 2. The Southern Ocean ecosystem occupies the circumpolar area between the continent of Antarctica and the Subtropical Front at approximately 401 S latitude; the Southern Ocean ecosystem has a subantarctic region from the Subtropical Front south to the Polar Front and an Antarctic region from the Polar Front to the Antarctic continent. 3. The equatorial upwelling ecosystem occupies an equatorial band from 51 N to 51 S and from South America westward to 1801 in the eastern and central Pacific Ocean. This region is often called the ‘‘cold tongue’’ because upwelling keeps these tropical waters surprisingly cool. In the Atlantic the 51 N to 51 S band of equatorial upwelling reaches from Africa across to South America. In the Indian
584
Summary of heat flux, stratification, nutrient, and light characteristics of ocean ecosystems
Oceanecosystema
Low-latitude gyre
Heat flux
Southern Ocean
Neutral; negative in Western Boundary Current Negative (ocean loses heat)
Equatorial upwelling
Positive (ocean gains heat)
Subarctic gyre
Seasonally positive and negative
Eastern Boundary Current Coastal upwelling
Positive
a
Positive
Stratification
Nutrient
Light
Strength
Duration
Level
Source
Level
Pattern
Strong
Permanent
Low ({Ks)b
Eddy diffusion, very weak convection
High (cEk)c
Continuous
Very weak, except strong when ice melts in summer Strong stratification following vertical transport Moderate stratification following winter mixing Medium
Seasonal
High (4KS), except Si(OH)4
Mixing and upwelling
Moderate in summer; low rest of year
Strongly seasonal
Permanent
High (4KS), except Si(OH)4
Upwelling and mixing
High (cEk)
Continuous
Seasonal convective mixing
High (4KS in winter; EKS rest of year)
Convective mixing and eddies
Low in winter ({Ek); moderate rest of year (EEk)
Seasonal
Permanent
Medium (4KS) High (cKS)
Moderate (4Ek), except in winter High (4Ek)
Seasonal
Continuous
Upwelling and lateral advection Upwelling
Strong stratification following vertical transport
See text for the location and boundaries of these ecosystems. Ks is the nutrient concentration at which nutrient uptake occurs at one-half the maximal rate; at concentrations oKs, uptake is limited. c Ek is the light level at which the photosynthetic rate is light saturated; at light levels oEk, photosynthesis is light limited. b
Ocean Ecosystems
Table 1
Weakly seasonal
Table 2
Summary of size, primary productivity, export, and limiting factors of ocean ecosystems
Ocean ecosystem
Sizea
Export ratioc
Factors limiting primary productivity
Factors limiting fish yield
Low primary productivity, small size of organisms and long food web Short duration of summer bloom
Area (m2 1012)
%
Primary productivity amount and pattern (mmol C m 2 day 1)b
Low-latitude gyre
164
52
35 continuously
Low; weak seasonality
Macronutrients
Southern Ocean
77
25
High in summer; very low in winter
Iron and light
Equatorial upwelling Subarctic gyre
22 22
7 7
Low export productivity Short duration of spring bloom
21
7
Low continuously Episodically high; moderate rest of year High in summer; moderate rest of year
Iron and silicate Depth of winter mixing
Eastern Boundary Current
Unclear, but iron is a factor, and light is also in winter
Unclear
6
2
120 summer mean; E35 in spring and fall; E0 in winter 90 continuously 150 spring bloom mean; E50 rest of year 150 summer mean; 75 rest of year; grades into gyre E35 300 continuously close to coast; grades into Eastern Boundary Current E150
High, but spatially variable
Iron
Small size of ecosystem
Coastal upwelling
a
Size was calculated for the world ocean exclusive of noncoastal upwelling, continental shelf regions; total area was 312 1012 m2. Based mainly on recent measurements made by the author in the Joint Global Ocean Flux Study. c Export ratio is the relative proportion of primary productivity that is exported vertically, horizontally, or to higher trophic levels. The maximum observed export ratios relative to total primary productivity are approximately 0.50. b
Ocean Ecosystems 585
586
Ocean Ecosystems
Table 3
Summary of biotic characteristics, the zooplankton component of ocean ecosystems
Ecosystem
Endemic spp.
Species richness
Species evenness
Variability of biomass
Size of organisms
Importance of protozoan micrograzers
Low-latitude gyre Southern Ocean Equatorial upwelling Subarctic gyre Eastern Boundary Current Coastal upwelling
Low High Moderate High Moderate Low
High Low High Low Moderate High
High Very low High Low Moderate Moderate
Low Very high (45 ) Low High (42 ) Low (o2 ) Moderate (E2 )
Small Large Small Moderate Moderate Large
Dominant Moderate to low Dominant Moderate to low Dominant to low Low
Ocean equatorial region there is no manifestation of this ecosystem because the upwelling, if present, does not extend upwards into the euphotic zone. 4. A subarctic gyre ecosystem is present in both the North Atlantic and the North Pacific in the region north of the Subtropical Front at approximately 401 N. In the Atlantic, the subarctic gyre ecosystem is bounded on the south by the North Atlantic Current and Subtropical Front, on the west by the Labrador Current, on the north by the East Greenland Current, and on the east by Europe. The North Atlantic subarctic gyre is a complex, but very welldelineated, ocean feature. The North Pacific subpolar gyre is bounded on the south at approximately 401 N by the North Pacific Current and the Subtropical Front, on the west by Siberia, on the north by the Aleutian Islands and Alaska, and on the east by Canada. 5. An Eastern Boundary Current (EBC) ecosystem is represented in each of the four great EBCs: the California Current off the west coast of the United States and Mexico, the Peru Current off Peru and Chile, the Canary Current off Northwest Africa, and the Benguela Current off Namibia and South Africa. These locations are characterized by a broad and weak equatorward-flowing current that usually extends offshore 400–600 km. Each of these ecosystems receives upwelled water from a coastal upwelling ecosystem and, in turn, exports water to the adjacent low-latitude ecosystem. 6. There are four great coastal upwelling ecosystems: Peru Current, Benguela Current, Canary Current, and California Current. This ecosystem is narrow but long; it extends for great distances along the west coasts of South America, North America, northwest Africa, and South Africa. This ecosystem is also present in the northwestern Arabian Sea, along the coasts of Oman and Yemen, where evidence of upwelled water is present up to 600 km off the Omani coast.
Characteristics of Ocean Ecosystems Shared Characteristics The following characteristics of ocean ecosystems are based on a 1974 synthesis by McGowan for the oceanic Pacific Ocean: They are few in number (only six). This small number of ocean ecosystems contrasts dramatically with the much larger
number of well-defined terrestrial, intertidal, and coastal habitats that occur in much smaller areas. The reason, of course, is that the fluid medium of the open ocean is fundamentally partitioned by the dominant physical processes that occur in an oceanic area, and there are only a few (six) of these overriding physical patterns. In contrast, terrestrial, intertidal, or coastal ecosystems are fundamentally partitioned by spatial or geographic features and there is a much larger number of unique geographic features associated with the solid surface of the earth’s crust. The same fundamental concept pervades all aspects of the comparison of oceanic biodiversity with that of terrestrial, intertidal, or coastal biodiversity. The fluid medium of the ocean is relatively homogeneous, and the boundaries that do exist are dynamic processes such as the presence or absence of winter convective mixing. It is obvious that dynamic processes of this nature form physical boundaries that permit much more biological exchange than the physical barrier associated with the solid earth. They are large relative to terrestrial, intertidal, or coastal ecosystems. The largest ocean ecosystem, the low-latitude gyre ecosystem, occupies 52% of the open ocean area of the world ocean. This ecosystem’s size is a simple reflection of the phenomenon that the earth has vast surface area occupied by the great circulation features called gyres which are slow-turning circular ocean current patterns set in motion ultimately by wind and the rotation of the earth. These gyres, together with the tropical warm pool regions and certain of the marginal seas, occupy a large portion of the earth’s surface. Because of the size of the low-latitude gyre ecosystem, it plays an important role in the global heat budget and in exchanges with the atmosphere, and it is the single most important ecosystem for understanding future global change. Ironically and unfortunately, it is one of the least understood ecosystems on Earth. Its size and remoteness from societal activity, especially direct economic activity, have led both scientific and political policymakers to assign the large low-latitude gyre ecosystem a low priority for scientific effort. On the other hand, the smallest ocean ecosystem, the coastal upwelling ecosystem, has received thorough, comprehensive, and multinational study during the past three decades. This ecosystem is small relative to other ocean systems; it has very strong physical, chemical, and biological processes; it has huge economic importance and considerable geological importance because much of the earth’s oil is assumed to have been formed in past upwelling ecosystems. Coastal upwelling ecosystems do have great societal importance, and the global effort to study and understand these
Ocean Ecosystems
important systems reflects well on the wisdom of scientific and political decision makers. They are geologically old (with the exception of coastal upwelling). Oceanic cores suggest that the defining oceanographic features have been in place for the past 200,000 years and perhaps for the past 1 million years. The coastal upwelling ecosystem, however, may have disappeared entirely during periods of lower sea level such as during the last Glacial Maximum approximately 18,000 years ago. They respond to climate but not to weather. Subtle and pervasive changes in mixed layer depth, the depth of convective mixing, or the strength of stratification cause profound changes in ocean ecosystems. On the other hand, strong storms or the passage of a violent hurricane cause no change that is detectable in the ocean ecosystem 2 weeks after the event. El Nin˜o events cause profound changes in equatorial upwelling, EBC, and coastal upwelling ecosystems, but when El Nin˜o ends the lower trophic levels of these ecosystems (phytoplankton, bacteria, microzooplankton, and mesozooplankton) return to their pre-El Nin˜o condition within 1 month. Higher trophic levels, of course, require several years to recover. They have considerable internal homogeneity, i.e., they tend to be monotonous. The key word is ‘‘internal.’’ There are clear changes in almost all biological properties when physical or oceanographic boundaries are crossed; however, if
System forcing functions
587
oceanographic boundaries are not crossed, biological properties will remain surprisingly similar over distances of thousands of kilometers. They are relatively undisturbed by anthropogenic processes compared to other ecosystems. The largest human disturbance thus far has been ruthless overfishing that removes top predators. The removal of these long-lived and slow-growing carnivores appears to set off a trophic cascade that changes the ecosystem even down to the level of nutrient regeneration by bacteria and protozoa. A second and more threatening change is that as the earth’s heat budget changes the processes of precipitation, winter mixing, stratification, and depth of the nutricline all change in such a way that there is less transport of nutrients into the euphotic zone. This change, already apparent in the North Pacific Ocean, will reduce productivity and, hence, the yield of food resources. The basic organization (Figure 1) is like that of other ecosystems. It appears that the basic assembly rules for all ecosystems are very similar. There has to be a large number of primary producers, somewhat fewer herbivores, still fewer carnivores, and many fewer top predators. There have to be efficient recyclers to return a portion of the nutrients back to the euphotic zone. In addition, the ecological ‘‘rules’’ regarding diversity, species composition, and adaptation are expressed similarly in terrestrial and ocean ecosystems.
System sinks and feedbacks
System inputs
Regeneration
Nekton
Fecal pellets
Burial
Coreolis Rotation
Predation
Grazing Detritus
Circulation
Mesozooplankton
Regeneration
Upwelling
Nutrient uptake
Phytoplankton (two sizes) Grazing
Eolian flux
Vertical transport
Euphotic zone nutrients
Sinking
Mixing
Sea ice melting
Wind stress Wind stress
Upper ocean dynamics
Microzooplankton
Photosynthesis
Light
Fishing
Heating Atmosphere
Deep nutrients Regeneration
Sediments Figure 1 Generalized organization of ocean ecosystems. Physical forcing and input functions are shown in dark gray, chemical and geologic input functions are white, and biological components are in light gray. Physical transfers of energy, momentum, or mass are shown by arrows with solid lines; biological transfers of mass and energy between living components are shown by arrows with dotted lines. The export process at the extreme right, labeled fishing, is a proxy for all processes that remove biomass from the euphotic zone of the represented ecosystem.
588
Ocean Ecosystems
Distinguishing Characteristics There are qualitative differences in the basic biological processes. The most dramatic illustration of this is the observation that processes limiting productivity, biomass, export, and yield are different in almost every one of these six ecosystems. Although our understanding of limiting factors may change, currently it appears that the low-latitude gyres are limited by fixed nitrogen, especially nitrate and ammonia; the Southern Ocean appears to be limited by iron and light; the equatorial upwelling ecosystem appears to be limited by iron and silicon; coastal upwelling ecosystem rates appear not to be limited by any macronutrient, but the space where optimal coastal upwelling occurs is highly constrained and iron supply is involved; the EBC ecosystem appears to be limited by light and iron in winter and by iron alone in the summer; and the subarctic gyre ecosystem appears to be limited by the depth of the winter mixed layer or convection. There are quantitative differences in basic processes such as primary productivity among different kinds of ocean ecosystems. One of the important milestones in the study and understanding of these differences was the work of John Ryther, who in 1969 published a work that provided a quantitative explanation of why fish yields vary by approximately 200-fold from the richest ocean ecosystems to the poorest. Variations in productivity, of course, are well-known from terrestrial ecosystems, but on land either aridity in deserts or freezing in polar regions is responsible for the low productivity of the poorest regions. Understanding why the benign low-latitude gyre ecosystem was so poor in fish production was much more difficult than understanding why productivity was low in deserts and polar regions. Part of the explanation was proposed in 1955 by Sverdrup, who said simply that the physical supply of nutrients to the euphotic zone is the reason for low fish yields in stratified ocean ecosystems. Ryther’s contribution amplified the physical explanation by considering the nature of the ecological processes that lead to fish production. First, Ryther estimated that approximately half the fish caught in the world are caught in coastal upwelling ecosystems, the smallest of the ocean ecosystems. Why? To begin, Sverdrup was correct: The physical processes of upwelling bring abundant nutrients to the surface layer, so primary productivity is very high in upwelling ecosystems. However, much more is involved. The phytoplankton that thrive in the rich coastal upwelling ecosystems are very largeFso large that some are eaten directly by fish. This means that in coastal upwelling ecosystems the food chain leading to fish is very short. Ryther estimated that half the fish diet was phytoplankton and half was zooplankton. On average, then, the length of the food chain leading to fish had 1.5 transfers: large phytoplankton to fish or large phytoplankton to zooplankton to fish. At each ecological transfer, a large portion of the energy of the food is used to support the organism and this portion cannot be passed up the food chain. The shortness of the food chain is a factor that multiplies the high nutrient effect. Ryther also noted that in the small and food-rich coastal upwelling ecosystem the fish and zooplankton did not have to work as hard to get food, so the efficiency of transfer was increased relative to that of a poor environment such as the low-latitude gyre. Ryther proposed that fish yields were high in the coastal upwelling ecosystem
because of high nutrients, high primary productivity, large size of the primary producers, short food chains with few transfers, and increased efficiency of transfer. These effects multiply each other, leading to very high yields of fish. For the same reason, the abundance of seabirds and marine mammals is also very high in the coastal upwelling ecosystem. The same arguments in reverse explain the low fish yields of the low-latitude gyre ecosystem. The other four ocean ecosystems range between these two extremes. The order of fish yield per unit area can be approximately estimated, from highest to lowest, as (1) coastal upwelling ecosystem, (2) subarctic gyres ecosystem, (3) EBC ecosystem, (4) Southern Ocean ecosystem, (5) equatorial upwelling ecosystem, and (6) low-latitude gyre ecosystem.
Ocean Ecosystems and Global Change Human intervention in the material cycles and trophic structure of ocean ecosystems may already have caused some changes. The interventions are so varied and play out at so many different scales that it is difficult to know how to describe their impact on future climate change. One approach is to focus on model studies of how anthropogenic ocean warming will affect ocean ecosystems. Considerable effort has gone into investigating how increased atmospheric carbon dioxide will affect the physical ocean–atmosphere system. Other effort has gone into determining how the ocean’s carbon dioxide system will behave in an anthropogenically warmed ocean. Today’s atmosphere–ocean climate models are fully coupled to an ocean carbon model, including a full carbon system and the biological transfers of the pelagic food web. Results of these model runs are dramatic and, as expected, they predict (i) a warming of global mean sea surface temperature by 2.5 1C, (ii) a slight decrease in light because of increased cloud cover, (iii) a large increase in vertical stratification because of increased precipitation which lowers salinity, and (iv) increased heating of the surface layer. The increase in stratification and decrease in convective mixing are predicted to cause a 37% reduction in the supply of nutrients to the euphotic layer. All physical processes that transport nutrients from the deep ocean reservoir to the surface layer are affected: upwelling, wind and tidal mixing, and convective mixing. Of the three classes of impactsFwarming, light, and increased stratificationFincreased stratification will have the major impact on ocean ecosystems. The changes predicted vary in intensity from one ocean ecosystem to another, but they are most severe in productive ecosystems such as the subantarctic gyre, equatorial upwelling, and coastal upwelling ecosystems. Very little change in nutrient supply is predicted for the world ocean’s five great low-latitude gyres. The predicted change is an expansion of the size of the world ocean’s most oligotrophic ecosystems: Approximately half of the ocean area will have decreased nutrient supply; the low-latitude gyres, the poorest half, will have no change; and a very small area (less than 5%) in the high Arctic and Antarctic will have an increase in nutrient flux to the surface.
Ocean Ecosystems
The El Nin˜o phenomenon has provided evidence of the biological consequences of reducing an ocean ecosystem nutrient supply. New populations that became established under the low nutrient conditions are healthy, highly diverse communities, but they are dramatically different. Another consequence of reduced nutrients is that the biological pump sequesters less carbon dioxide. This change will feed back into the global carbon system to accelerate the increase in concentration of atmospheric carbon dioxide. A reduction of new nutrients by 37% will have a strong impact on both the quantity and the kind of fish present. Because the increase will occur in what are now rich fishing regions, the coastal upwelling and subarctic gyre, the societal impact will be larger than the biological impact. A change of 37% in new nutrients is significant, but it is not enough to destroy ocean ecosystems. The current gradient in nutrient flux from the low-latitude gyre ecosystem to a coastal upwelling ecosystem region is more than a factor of 10. The predicted changes will reduce the yield from the world’s rich fishing banks, but the expanded low-latitude gyre ecosystems should maintain their ecological integrity.
See also: Climate Change and Ecology, Synergism of. Ecosystem, Concept of. Marine Ecosystems. Pelagic Ecosystems. Plankton, Status and Role of
589
References Barber RT (1988) Ocean basin ecosystems. In: Pomeroy LR and Alberts JJ (eds.) Concepts of Ecosystem Ecology, pp. 171–193. New York: Springer-Verlag. Barber RT and Smith RL (1981) Coastal upwelling ecosystems. In: Longhurst A (ed.) Analysis of Marine Ecosystems, pp. 31–68. San Diego: Academic Press. Karl DM (1999) A sea of change: Biogeochemical variability in the North Pacific Subtropical Gyre. Ecosystems 2: 181–214. Longhurst A (1998) Ecological Geography of the Sea. San Diego: Academic Press. McGowan JA (1974) The nature of oceanic ecosystems. In: Miller C (ed.) The Biology of the Oceanic Pacific. Proceedings of the 33rd Annual Biology Colloquium, pp. 9–28. Corvallis: Oregon State Univ. Press. Odum EP (1969) The strategy of ecosystem development. Science 164: 262–270. Odum EP (1977) The emergence of ecology as a new integrative discipline. Science 195: 1289–1293. Odum EP and Odum HT (1959) Fundamentals of Ecology. Philadelphia: Saunders. Pomeroy LR and Alberts JJ (eds.) (1988) Ecological Studies 67, Concepts of Ecosystem Ecology, A Comparative View. New York: Springer-Verlag. Ryther JH (1969) Photosynthesis and fish production in the sea. Science 166: 72–76. Sherman K and Alexander LM (eds.) (1986) Variability and Management of Large Marine Ecosystems, AAAs Selected Symposium No. 99. Boulder, CO: Westview. Steele JH (1985) A comparison of terrestrial and marine ecological systems. Nature 313: 355–358. Steele JH (1998) From carbon flux to regime shift. Fisheries Oceanogr. 7: 176–181. Sverdrup HU (1955) The place of physical oceanography in oceanographic research. J. Mar. Res. 14: 287. Tomczak M and Godfrey JS (1994) Regional Oceanography: An Introduction. Oxford: Pergamon.
Glossary Convective mixing Vertical mixing produced by the increasing density of a fluid in the upper layer, especially during winter in temperate and polar regions. Euphotic zone The surface layer of the ocean in which there is adequate light for net positive photosynthesis. Nutrients Dissolved mineral salts necessary for primary productivity and phytoplankton growth: Macronutrients are phosphate, nitrate, and silicate; micronutrients are iron, zinc, manganese, and other trace metals. Phytoplankton Photosynthetic, usually single-celled, plants that drift with ocean currents. Pycnocline The layer in which density changes most rapidly with depth and separates the surface mixed layer from the deep ocean waters. Southern Ocean The circumpolar ocean in the Southern Hemisphere between the Subtropical Front and the continent of Antarctica.
History of the Ecosystem Concept Natural scientists have long recognized that beyond populations or communities there is a higher level of organization, but it has been difficult for physical and biological scientists to reach consensus about its nature. The origins of the ecosystem concept are indistinct and, indeed, involved polemics between some of the principals. Ernst Haeckel (1838–1919) coined the word ‘‘ecology’’ and he envisioned ecology as a science that studies the environment as a stage within which selection pressures and adaptations affect the evolution of species. Haeckel brought the physical environment to the forefront, but he was interested primarily in evolution, which acts on species or genetically interacting populations. Victor Hensen (1835–1924) coined the word ‘‘plankton’’ to describe the organisms that drift in the ocean following the paths dictated by currents and mixing. Hensen was interested in the integrated working of natural systems, the subject today called ‘‘ecosystem science.’’ He was particularly interested in the functional behavior of ocean systems from the smallest phytoplankton up
Encyclopedia of Biodiversity, Volume 5
Stratification The formation of distinct layers with different densities; stratification inhibits mixing. Subpolar Pertaining to the regions between the polar and temperate zones, but for the oceans the boundaries are the Subtropical Front and the Polar Front. Subtropical Pertaining to the regions which, under the influence of the trade winds, are permanently stratified. Upwelling Upward vertical movement of water through the bottom of the surface mixed layer produced by a divergence at the surface. Zooplankton Animals that float or drift with ocean currents: Microzooplankton are protozoan plankton that graze on small phytoplankton; mesozooplankton are crustaceans that graze on larger phytoplankton such as diatoms.
through the food web and leading finally to fish, birds, and marine mammals. Hensen and Haeckel had a long professional dispute which seems ironic today because the eventual synthesis of their ideas by others led indirectly to the ecosystem concept. The linguistic route to the modern word ‘‘ecosystem’’ was tortuous. From 1877 to 1939 the words ‘‘biocoenose,’’ ‘‘microcosm,’’ ‘‘naturkomplex,’’ ‘‘holocen,’’ and ‘‘biosystem’’ were all proposed as names for this idea. In 1935 an English scientist, Tansley, introduced the neologism ‘‘ecosystem’’ and this term rapidly eclipsed competing names, but ecosystem science was not a well-organized endeavor until 1959, when Eugene Odum and Howard Odum published a groundbreaking textbook, Fundamentals of Ecology. There were many treatises on ecology before the Odums’ book, but their book stressed the principle that the ecosystem (not the population or community) is the fundamental unit of ecology and indeed of biology. The 1959 text, which develops this concept in each of its chapters, has remained in print for many years. Although Eugene Odum’s definition of ecosystem (given at the beginning of this article) is widely accepted, the principle of the ecosystem as the
http://dx.doi.org/10.1016/B978-0-12-384719-5.00188-X
581
582
Ocean Ecosystems
fundamental unit of ecology is controversial. Populations, communities, adaptation, and evolution have all been explosively successful research areas. Ecosystem research, because of its inherent complexity and requirement for interdisciplinary work, has not enjoyed the success of the more reductionist areas of ecological research. The study of ecosystems is usually supported by encouraging words, but our institutions and agencies, which are organized along disciplinary lines, have trouble coping with a science composed of equal parts of physics, chemistry, and biology. Ecosystem studies are placed with the life sciences, in which there is little institutional or agency enthusiasm for the complex and expensive atmospheric and physical oceanographic work required by the study of ocean ecosystems.
Utility of the Concept The ecosystem concept was originally developed for terrestrial, intertidal, and benthic habitats, in which ecological succession on timescales of 10–100 years is an obvious and salient characteristic of the ecosystem. Succession on this timescale is difficult or impossible to detect in the fluid medium of pelagic ocean ecosystems. In the ocean, winds, mixing, and currents appear to reset the successional time clock to time zero each annual cycle or, perhaps, with each storm or passage of a front or large eddy. It has been shown that there are low-frequency (over the timescale of 10–100 years) changes in species abundance and community structure. These biological changes are sometimes so far reaching that they are called ‘‘regime shifts,’’ the term used by John Steele in 1998, but they are not functionally analogous to succession, which is (i) orderly and directional, (ii) involving biological modification of the physical environment, and (iii) resulting in a more stable climax community (Odum, 1969). Succession in ocean ecosystems is dramatically evident on the day to month timescale after spring stratification in temperate and subpolar waters or after an episode of upwelling. This short timescale succession, which appears to be cyclic, is not unidirectional and definitely not orderly. If long-term directional succession does not appear to occur in pelagic ocean ecosystems and succession is a central tenet of ecosystem theory, isn’t the very existence of ocean ecosystems in doubt? The answer is that ocean ecosystems, as Steele (1985) noted, are clearly different from terrestrial, aquatic, intertidal, or benthic ecosystems, but ocean ecosystems meet most of the requirements of the definition set forth by Eugene Odum. The ocean ecosystems described here all have a characteristic and distinct trophic structure, characteristic and distinct material cycles, some degree of internal homogeneity and commonality, and definable hydrographic boundaries. There is considerable heuristic power in the ecosystem concept because understanding gained in one ocean ecosystem can be used to predict the response of another ecosystem of the same kind that is geographically distinct from it. This predictive power is perhaps the greatest benefit of ecosystem theory and provides evidence that each distinct kind of ocean ecosystem has characteristics that can be generalized and used in prediction. Pomeroy and Alberts (1988) emphasized that the concept involves emergence of new properties. One consequence of the hierarchic organization of ecosystems is that as
components (both biotic and abiotic) are integrated into a larger functional unit, new properties emerge that cannot be detected by study of the component populations or processes, no matter how thorough the reductionist study. This aspect of ecosystem theory makes the concept useful, even necessary, for predictive understanding of ocean ecosystems. Determining specific emergent properties of a system as large as ocean ecosystems is difficult. Ocean ecosystem spatial domains of thousands of kilometers are difficult to sample adequately with ships; however, with satellites that measure wind, ocean temperature, ocean currents (from sea surface topography), and phytoplankton biomass, a new era has begun in understanding ocean ecosystems. Are there appropriate benefits to justify the large societal investment in remote sensing and data handling required to achieve this new level of understanding? One benefit deals with fisheries management. The historic approach to management of these fisheries has involved analysis of local populations and environment. It has become clear that ecosystem properties, especially physical conditions, operating on scales much larger than the range of the exploited population, may be responsible for changes in reproductive success and adult abundance. In this context, a valuable societal payoff of understanding ocean ecosystems is improved ability to predict local variations in living resources as shown by Sherman and others in 1986. Odum repeatedly emphasized that ecosystem science and economics are parallel disciplines and expressed regret that they are not perceived as such, particularly by the economic community. Investors and political leaders making policy decisions on resource development need an understanding of the probability, frequency, and intensity of natural variability for realistic economic decision making. In this context, economics and the ecosystem concept are closely related: By understanding the variability of ocean ecosystems, decision makers will also understand that variation in marine resources is a normal and inevitable characteristic that must be accommodated in economic plans. This kind of resource-related benefit, although important, is not the only societal benefit that will accrue. Odum (1977) said, ‘‘It is the properties of the large-scale integrated systems that hold solutions to most of the longrange problems of society.’’ Although few in the scientific community recognized the wisdom of this comment two decades ago, the validity of Odum’s prediction has now been well demonstrated. For example, carbon dioxide modification of the planet’s heat budget, and therefore climate, is a phenomenon that can be understood only if the emergent properties of large-scale biogeochemical systems are understood.
Partitioning the Ocean into Natural Functioning Units Central Problem The central problem for the lower trophic level of ocean ecosystems is obtaining light (energy) and inorganic nutrients (mass). Odum’s definition requires that for a functioning system to be a distinct ecosystem it must possess characteristic trophic structure and material cycles. That is, how one kind of ocean ecosystem captures light and passes that energy on in
Ocean Ecosystems
583
the form of primary productivity, secondary productivity, and so forth is different from how another kind of ocean ecosystem processes and transfers its energy. Likewise, how mass (C, N, P, and Si), initially in the form of inorganic compounds, is taken up and transferred through the food web and eventually released back to the environment is different and very poorly understood. What controls the supply of light and nutrients to an ocean ecosystem? Sverdrup in 1955 was the first oceanographer to note that the spatial and temporal patterns of physical processes, particularly the seasonal patterns of mixing, stratification, and upwelling as well as the seasonal pattern of irradiance, control the patterns of biological organization. The division of ocean ecosystems into six distinct types is based fundamentally on the pioneering work of Sverdrup and that of others in the intervening years.
Using these criteria, Longhurst partitioned the ocean into four major biomes: the westerlies biome, in which convective mixing and stratification are forced largely by the strong seasonal progression of winds, irradiance, and heat flux; the trades biome, in which upwelling and mixing are forced on the oceanbasin scale by both local and remote wind forcing; the polar biome, in which there is no significant thermal stratification and mixed layer depth is constrained by a surface low-salinity layer which forms each summer as the marginal ice melts; and the coastal biome, in which coastal processes such as tides or currents break down stratification and force mixing. Longhurst’s (1998) partitioning of the ocean into four biomes and 55 provinces is a significant accomplishment. Eventually, his ideas will be merged with the ecosystem concept to produce an internally consistent hierarchy of biomes, ecosystems, and provinces.
Biome Concept
Ecosystem Concept
Longhurst (1998) presented a scholarly discussion of the attempt to partition the oceans into natural functional provinces. He described with considerable elan a regional ecology of the oceans which is clearly distinct from partitioning the ocean into ecosystems. The difference can be illustrated using one of the best described ocean ecosystems, the coastal upwelling ecosystem. It is well accepted that there is one coastal upwelling ecosystem and that it is replicated at five coastal regions in the world ocean: the California Current off California and Mexico, the Peru Current off Peru and Chile, the Benguela Current, the Canary Current, and the Arabian Sea off Oman and Yemen. Longhurst recognized the same regions but counted them as five distinct ocean ‘‘provinces.’’ The ecosystem concept emphasizes that there are geographic ocean regions that have much in common; ecological geography emphasizes the discreteness or autonomy of various ocean provinces. Both concepts are useful. Longhurst (1998) proposed that the following information is required to define the functional nature of an ocean region:
Using the formal Odum definition of ecosystem, six more or less well-defined ocean ecosystems can be delineated (Table 1), but note that the boundaries of these ecosystems are usually oceanographic, not geographic, features. (See Tomczak and Godfrey (1994) for a description of these oceanographic features). Although it is possible to describe approximately where these ecosystems exist, the actual domain is determined by dynamic processes such as fronts where two kinds of ocean water converge. The approach is to define the characteristic trophic structure and material cycles by applying the same analysis to each system. Each of the six systems has a different combination of stratification, nutrient supply (Table 1), primary productivity (Table 2), and biotic characteristics (Table 3). This analysis indicates that there are six distinct ocean systems that meet Odum’s criteria:
Latitude, which determines if wind stress induces mixing or wind-driven advection Depth of water because stratification may be broken down in shallow seas by tidal mixing Proximity to coastline, which determines the effects of terrestrial runoff, river discharge, and release of nutrients (especially Fe) from sediments Seasonal irradiance, which forces photosynthesis, stratification, and freezing or melting of ice Winds, which force mixing or upwelling of subsurface waters and their nutrients up to the euphotic zone Precipitation, which may induce strong stratification by making the surface layer less salty Nutricline depth, which modifies the vertical flux of nutrients by wind mixing and upwelling Strength of the vertical nutrient gradient, which determines the magnitude of the upward nutrient flux External source of iron, because insufficient iron may limit uptake of macronutrients, phytoplankton growth, or primary productivity
1. A low-latitude gyre ecosystem is present in each of the five great low-latitude gyresFNorth Atlantic, South Atlantic, North Pacific, South Pacific, and South Indian OceansFas well as in the warm pool of the western Pacific Ocean, the equatorial Indian Ocean, and in large marginal seas such as the Mediterranean Sea and the Gulf of Mexico. This ecosystem is also present in the Western Boundary Current regions between the western edge of each of the five great gyres and the coastal waters of the adjacent continent. 2. The Southern Ocean ecosystem occupies the circumpolar area between the continent of Antarctica and the Subtropical Front at approximately 401 S latitude; the Southern Ocean ecosystem has a subantarctic region from the Subtropical Front south to the Polar Front and an Antarctic region from the Polar Front to the Antarctic continent. 3. The equatorial upwelling ecosystem occupies an equatorial band from 51 N to 51 S and from South America westward to 1801 in the eastern and central Pacific Ocean. This region is often called the ‘‘cold tongue’’ because upwelling keeps these tropical waters surprisingly cool. In the Atlantic the 51 N to 51 S band of equatorial upwelling reaches from Africa across to South America. In the Indian
584
Summary of heat flux, stratification, nutrient, and light characteristics of ocean ecosystems
Oceanecosystema
Low-latitude gyre
Heat flux
Southern Ocean
Neutral; negative in Western Boundary Current Negative (ocean loses heat)
Equatorial upwelling
Positive (ocean gains heat)
Subarctic gyre
Seasonally positive and negative
Eastern Boundary Current Coastal upwelling
Positive
a
Positive
Stratification
Nutrient
Light
Strength
Duration
Level
Source
Level
Pattern
Strong
Permanent
Low ({Ks)b
Eddy diffusion, very weak convection
High (cEk)c
Continuous
Very weak, except strong when ice melts in summer Strong stratification following vertical transport Moderate stratification following winter mixing Medium
Seasonal
High (4KS), except Si(OH)4
Mixing and upwelling
Moderate in summer; low rest of year
Strongly seasonal
Permanent
High (4KS), except Si(OH)4
Upwelling and mixing
High (cEk)
Continuous
Seasonal convective mixing
High (4KS in winter; EKS rest of year)
Convective mixing and eddies
Low in winter ({Ek); moderate rest of year (EEk)
Seasonal
Permanent
Medium (4KS) High (cKS)
Moderate (4Ek), except in winter High (4Ek)
Seasonal
Continuous
Upwelling and lateral advection Upwelling
Strong stratification following vertical transport
See text for the location and boundaries of these ecosystems. Ks is the nutrient concentration at which nutrient uptake occurs at one-half the maximal rate; at concentrations oKs, uptake is limited. c Ek is the light level at which the photosynthetic rate is light saturated; at light levels oEk, photosynthesis is light limited. b
Ocean Ecosystems
Table 1
Weakly seasonal
Table 2
Summary of size, primary productivity, export, and limiting factors of ocean ecosystems
Ocean ecosystem
Sizea
Export ratioc
Factors limiting primary productivity
Factors limiting fish yield
Low primary productivity, small size of organisms and long food web Short duration of summer bloom
Area (m2 1012)
%
Primary productivity amount and pattern (mmol C m 2 day 1)b
Low-latitude gyre
164
52
35 continuously
Low; weak seasonality
Macronutrients
Southern Ocean
77
25
High in summer; very low in winter
Iron and light
Equatorial upwelling Subarctic gyre
22 22
7 7
Low export productivity Short duration of spring bloom
21
7
Low continuously Episodically high; moderate rest of year High in summer; moderate rest of year
Iron and silicate Depth of winter mixing
Eastern Boundary Current
Unclear, but iron is a factor, and light is also in winter
Unclear
6
2
120 summer mean; E35 in spring and fall; E0 in winter 90 continuously 150 spring bloom mean; E50 rest of year 150 summer mean; 75 rest of year; grades into gyre E35 300 continuously close to coast; grades into Eastern Boundary Current E150
High, but spatially variable
Iron
Small size of ecosystem
Coastal upwelling
a
Size was calculated for the world ocean exclusive of noncoastal upwelling, continental shelf regions; total area was 312 1012 m2. Based mainly on recent measurements made by the author in the Joint Global Ocean Flux Study. c Export ratio is the relative proportion of primary productivity that is exported vertically, horizontally, or to higher trophic levels. The maximum observed export ratios relative to total primary productivity are approximately 0.50. b
Ocean Ecosystems 585
586
Ocean Ecosystems
Table 3
Summary of biotic characteristics, the zooplankton component of ocean ecosystems
Ecosystem
Endemic spp.
Species richness
Species evenness
Variability of biomass
Size of organisms
Importance of protozoan micrograzers
Low-latitude gyre Southern Ocean Equatorial upwelling Subarctic gyre Eastern Boundary Current Coastal upwelling
Low High Moderate High Moderate Low
High Low High Low Moderate High
High Very low High Low Moderate Moderate
Low Very high (45 ) Low High (42 ) Low (o2 ) Moderate (E2 )
Small Large Small Moderate Moderate Large
Dominant Moderate to low Dominant Moderate to low Dominant to low Low
Ocean equatorial region there is no manifestation of this ecosystem because the upwelling, if present, does not extend upwards into the euphotic zone. 4. A subarctic gyre ecosystem is present in both the North Atlantic and the North Pacific in the region north of the Subtropical Front at approximately 401 N. In the Atlantic, the subarctic gyre ecosystem is bounded on the south by the North Atlantic Current and Subtropical Front, on the west by the Labrador Current, on the north by the East Greenland Current, and on the east by Europe. The North Atlantic subarctic gyre is a complex, but very welldelineated, ocean feature. The North Pacific subpolar gyre is bounded on the south at approximately 401 N by the North Pacific Current and the Subtropical Front, on the west by Siberia, on the north by the Aleutian Islands and Alaska, and on the east by Canada. 5. An Eastern Boundary Current (EBC) ecosystem is represented in each of the four great EBCs: the California Current off the west coast of the United States and Mexico, the Peru Current off Peru and Chile, the Canary Current off Northwest Africa, and the Benguela Current off Namibia and South Africa. These locations are characterized by a broad and weak equatorward-flowing current that usually extends offshore 400–600 km. Each of these ecosystems receives upwelled water from a coastal upwelling ecosystem and, in turn, exports water to the adjacent low-latitude ecosystem. 6. There are four great coastal upwelling ecosystems: Peru Current, Benguela Current, Canary Current, and California Current. This ecosystem is narrow but long; it extends for great distances along the west coasts of South America, North America, northwest Africa, and South Africa. This ecosystem is also present in the northwestern Arabian Sea, along the coasts of Oman and Yemen, where evidence of upwelled water is present up to 600 km off the Omani coast.
Characteristics of Ocean Ecosystems Shared Characteristics The following characteristics of ocean ecosystems are based on a 1974 synthesis by McGowan for the oceanic Pacific Ocean: They are few in number (only six). This small number of ocean ecosystems contrasts dramatically with the much larger
number of well-defined terrestrial, intertidal, and coastal habitats that occur in much smaller areas. The reason, of course, is that the fluid medium of the open ocean is fundamentally partitioned by the dominant physical processes that occur in an oceanic area, and there are only a few (six) of these overriding physical patterns. In contrast, terrestrial, intertidal, or coastal ecosystems are fundamentally partitioned by spatial or geographic features and there is a much larger number of unique geographic features associated with the solid surface of the earth’s crust. The same fundamental concept pervades all aspects of the comparison of oceanic biodiversity with that of terrestrial, intertidal, or coastal biodiversity. The fluid medium of the ocean is relatively homogeneous, and the boundaries that do exist are dynamic processes such as the presence or absence of winter convective mixing. It is obvious that dynamic processes of this nature form physical boundaries that permit much more biological exchange than the physical barrier associated with the solid earth. They are large relative to terrestrial, intertidal, or coastal ecosystems. The largest ocean ecosystem, the low-latitude gyre ecosystem, occupies 52% of the open ocean area of the world ocean. This ecosystem’s size is a simple reflection of the phenomenon that the earth has vast surface area occupied by the great circulation features called gyres which are slow-turning circular ocean current patterns set in motion ultimately by wind and the rotation of the earth. These gyres, together with the tropical warm pool regions and certain of the marginal seas, occupy a large portion of the earth’s surface. Because of the size of the low-latitude gyre ecosystem, it plays an important role in the global heat budget and in exchanges with the atmosphere, and it is the single most important ecosystem for understanding future global change. Ironically and unfortunately, it is one of the least understood ecosystems on Earth. Its size and remoteness from societal activity, especially direct economic activity, have led both scientific and political policymakers to assign the large low-latitude gyre ecosystem a low priority for scientific effort. On the other hand, the smallest ocean ecosystem, the coastal upwelling ecosystem, has received thorough, comprehensive, and multinational study during the past three decades. This ecosystem is small relative to other ocean systems; it has very strong physical, chemical, and biological processes; it has huge economic importance and considerable geological importance because much of the earth’s oil is assumed to have been formed in past upwelling ecosystems. Coastal upwelling ecosystems do have great societal importance, and the global effort to study and understand these
Ocean Ecosystems
important systems reflects well on the wisdom of scientific and political decision makers. They are geologically old (with the exception of coastal upwelling). Oceanic cores suggest that the defining oceanographic features have been in place for the past 200,000 years and perhaps for the past 1 million years. The coastal upwelling ecosystem, however, may have disappeared entirely during periods of lower sea level such as during the last Glacial Maximum approximately 18,000 years ago. They respond to climate but not to weather. Subtle and pervasive changes in mixed layer depth, the depth of convective mixing, or the strength of stratification cause profound changes in ocean ecosystems. On the other hand, strong storms or the passage of a violent hurricane cause no change that is detectable in the ocean ecosystem 2 weeks after the event. El Nin˜o events cause profound changes in equatorial upwelling, EBC, and coastal upwelling ecosystems, but when El Nin˜o ends the lower trophic levels of these ecosystems (phytoplankton, bacteria, microzooplankton, and mesozooplankton) return to their pre-El Nin˜o condition within 1 month. Higher trophic levels, of course, require several years to recover. They have considerable internal homogeneity, i.e., they tend to be monotonous. The key word is ‘‘internal.’’ There are clear changes in almost all biological properties when physical or oceanographic boundaries are crossed; however, if
System forcing functions
587
oceanographic boundaries are not crossed, biological properties will remain surprisingly similar over distances of thousands of kilometers. They are relatively undisturbed by anthropogenic processes compared to other ecosystems. The largest human disturbance thus far has been ruthless overfishing that removes top predators. The removal of these long-lived and slow-growing carnivores appears to set off a trophic cascade that changes the ecosystem even down to the level of nutrient regeneration by bacteria and protozoa. A second and more threatening change is that as the earth’s heat budget changes the processes of precipitation, winter mixing, stratification, and depth of the nutricline all change in such a way that there is less transport of nutrients into the euphotic zone. This change, already apparent in the North Pacific Ocean, will reduce productivity and, hence, the yield of food resources. The basic organization (Figure 1) is like that of other ecosystems. It appears that the basic assembly rules for all ecosystems are very similar. There has to be a large number of primary producers, somewhat fewer herbivores, still fewer carnivores, and many fewer top predators. There have to be efficient recyclers to return a portion of the nutrients back to the euphotic zone. In addition, the ecological ‘‘rules’’ regarding diversity, species composition, and adaptation are expressed similarly in terrestrial and ocean ecosystems.
System sinks and feedbacks
System inputs
Regeneration
Nekton
Fecal pellets
Burial
Coreolis Rotation
Predation
Grazing Detritus
Circulation
Mesozooplankton
Regeneration
Upwelling
Nutrient uptake
Phytoplankton (two sizes) Grazing
Eolian flux
Vertical transport
Euphotic zone nutrients
Sinking
Mixing
Sea ice melting
Wind stress Wind stress
Upper ocean dynamics
Microzooplankton
Photosynthesis
Light
Fishing
Heating Atmosphere
Deep nutrients Regeneration
Sediments Figure 1 Generalized organization of ocean ecosystems. Physical forcing and input functions are shown in dark gray, chemical and geologic input functions are white, and biological components are in light gray. Physical transfers of energy, momentum, or mass are shown by arrows with solid lines; biological transfers of mass and energy between living components are shown by arrows with dotted lines. The export process at the extreme right, labeled fishing, is a proxy for all processes that remove biomass from the euphotic zone of the represented ecosystem.
588
Ocean Ecosystems
Distinguishing Characteristics There are qualitative differences in the basic biological processes. The most dramatic illustration of this is the observation that processes limiting productivity, biomass, export, and yield are different in almost every one of these six ecosystems. Although our understanding of limiting factors may change, currently it appears that the low-latitude gyres are limited by fixed nitrogen, especially nitrate and ammonia; the Southern Ocean appears to be limited by iron and light; the equatorial upwelling ecosystem appears to be limited by iron and silicon; coastal upwelling ecosystem rates appear not to be limited by any macronutrient, but the space where optimal coastal upwelling occurs is highly constrained and iron supply is involved; the EBC ecosystem appears to be limited by light and iron in winter and by iron alone in the summer; and the subarctic gyre ecosystem appears to be limited by the depth of the winter mixed layer or convection. There are quantitative differences in basic processes such as primary productivity among different kinds of ocean ecosystems. One of the important milestones in the study and understanding of these differences was the work of John Ryther, who in 1969 published a work that provided a quantitative explanation of why fish yields vary by approximately 200-fold from the richest ocean ecosystems to the poorest. Variations in productivity, of course, are well-known from terrestrial ecosystems, but on land either aridity in deserts or freezing in polar regions is responsible for the low productivity of the poorest regions. Understanding why the benign low-latitude gyre ecosystem was so poor in fish production was much more difficult than understanding why productivity was low in deserts and polar regions. Part of the explanation was proposed in 1955 by Sverdrup, who said simply that the physical supply of nutrients to the euphotic zone is the reason for low fish yields in stratified ocean ecosystems. Ryther’s contribution amplified the physical explanation by considering the nature of the ecological processes that lead to fish production. First, Ryther estimated that approximately half the fish caught in the world are caught in coastal upwelling ecosystems, the smallest of the ocean ecosystems. Why? To begin, Sverdrup was correct: The physical processes of upwelling bring abundant nutrients to the surface layer, so primary productivity is very high in upwelling ecosystems. However, much more is involved. The phytoplankton that thrive in the rich coastal upwelling ecosystems are very largeFso large that some are eaten directly by fish. This means that in coastal upwelling ecosystems the food chain leading to fish is very short. Ryther estimated that half the fish diet was phytoplankton and half was zooplankton. On average, then, the length of the food chain leading to fish had 1.5 transfers: large phytoplankton to fish or large phytoplankton to zooplankton to fish. At each ecological transfer, a large portion of the energy of the food is used to support the organism and this portion cannot be passed up the food chain. The shortness of the food chain is a factor that multiplies the high nutrient effect. Ryther also noted that in the small and food-rich coastal upwelling ecosystem the fish and zooplankton did not have to work as hard to get food, so the efficiency of transfer was increased relative to that of a poor environment such as the low-latitude gyre. Ryther proposed that fish yields were high in the coastal upwelling ecosystem
because of high nutrients, high primary productivity, large size of the primary producers, short food chains with few transfers, and increased efficiency of transfer. These effects multiply each other, leading to very high yields of fish. For the same reason, the abundance of seabirds and marine mammals is also very high in the coastal upwelling ecosystem. The same arguments in reverse explain the low fish yields of the low-latitude gyre ecosystem. The other four ocean ecosystems range between these two extremes. The order of fish yield per unit area can be approximately estimated, from highest to lowest, as (1) coastal upwelling ecosystem, (2) subarctic gyres ecosystem, (3) EBC ecosystem, (4) Southern Ocean ecosystem, (5) equatorial upwelling ecosystem, and (6) low-latitude gyre ecosystem.
Ocean Ecosystems and Global Change Human intervention in the material cycles and trophic structure of ocean ecosystems may already have caused some changes. The interventions are so varied and play out at so many different scales that it is difficult to know how to describe their impact on future climate change. One approach is to focus on model studies of how anthropogenic ocean warming will affect ocean ecosystems. Considerable effort has gone into investigating how increased atmospheric carbon dioxide will affect the physical ocean–atmosphere system. Other effort has gone into determining how the ocean’s carbon dioxide system will behave in an anthropogenically warmed ocean. Today’s atmosphere–ocean climate models are fully coupled to an ocean carbon model, including a full carbon system and the biological transfers of the pelagic food web. Results of these model runs are dramatic and, as expected, they predict (i) a warming of global mean sea surface temperature by 2.5 1C, (ii) a slight decrease in light because of increased cloud cover, (iii) a large increase in vertical stratification because of increased precipitation which lowers salinity, and (iv) increased heating of the surface layer. The increase in stratification and decrease in convective mixing are predicted to cause a 37% reduction in the supply of nutrients to the euphotic layer. All physical processes that transport nutrients from the deep ocean reservoir to the surface layer are affected: upwelling, wind and tidal mixing, and convective mixing. Of the three classes of impactsFwarming, light, and increased stratificationFincreased stratification will have the major impact on ocean ecosystems. The changes predicted vary in intensity from one ocean ecosystem to another, but they are most severe in productive ecosystems such as the subantarctic gyre, equatorial upwelling, and coastal upwelling ecosystems. Very little change in nutrient supply is predicted for the world ocean’s five great low-latitude gyres. The predicted change is an expansion of the size of the world ocean’s most oligotrophic ecosystems: Approximately half of the ocean area will have decreased nutrient supply; the low-latitude gyres, the poorest half, will have no change; and a very small area (less than 5%) in the high Arctic and Antarctic will have an increase in nutrient flux to the surface.
Ocean Ecosystems
The El Nin˜o phenomenon has provided evidence of the biological consequences of reducing an ocean ecosystem nutrient supply. New populations that became established under the low nutrient conditions are healthy, highly diverse communities, but they are dramatically different. Another consequence of reduced nutrients is that the biological pump sequesters less carbon dioxide. This change will feed back into the global carbon system to accelerate the increase in concentration of atmospheric carbon dioxide. A reduction of new nutrients by 37% will have a strong impact on both the quantity and the kind of fish present. Because the increase will occur in what are now rich fishing regions, the coastal upwelling and subarctic gyre, the societal impact will be larger than the biological impact. A change of 37% in new nutrients is significant, but it is not enough to destroy ocean ecosystems. The current gradient in nutrient flux from the low-latitude gyre ecosystem to a coastal upwelling ecosystem region is more than a factor of 10. The predicted changes will reduce the yield from the world’s rich fishing banks, but the expanded low-latitude gyre ecosystems should maintain their ecological integrity.
See also: Climate Change and Ecology, Synergism of. Ecosystem, Concept of. Marine Ecosystems. Pelagic Ecosystems. Plankton, Status and Role of
589
References Barber RT (1988) Ocean basin ecosystems. In: Pomeroy LR and Alberts JJ (eds.) Concepts of Ecosystem Ecology, pp. 171–193. New York: Springer-Verlag. Barber RT and Smith RL (1981) Coastal upwelling ecosystems. In: Longhurst A (ed.) Analysis of Marine Ecosystems, pp. 31–68. San Diego: Academic Press. Karl DM (1999) A sea of change: Biogeochemical variability in the North Pacific Subtropical Gyre. Ecosystems 2: 181–214. Longhurst A (1998) Ecological Geography of the Sea. San Diego: Academic Press. McGowan JA (1974) The nature of oceanic ecosystems. In: Miller C (ed.) The Biology of the Oceanic Pacific. Proceedings of the 33rd Annual Biology Colloquium, pp. 9–28. Corvallis: Oregon State Univ. Press. Odum EP (1969) The strategy of ecosystem development. Science 164: 262–270. Odum EP (1977) The emergence of ecology as a new integrative discipline. Science 195: 1289–1293. Odum EP and Odum HT (1959) Fundamentals of Ecology. Philadelphia: Saunders. Pomeroy LR and Alberts JJ (eds.) (1988) Ecological Studies 67, Concepts of Ecosystem Ecology, A Comparative View. New York: Springer-Verlag. Ryther JH (1969) Photosynthesis and fish production in the sea. Science 166: 72–76. Sherman K and Alexander LM (eds.) (1986) Variability and Management of Large Marine Ecosystems, AAAs Selected Symposium No. 99. Boulder, CO: Westview. Steele JH (1985) A comparison of terrestrial and marine ecological systems. Nature 313: 355–358. Steele JH (1998) From carbon flux to regime shift. Fisheries Oceanogr. 7: 176–181. Sverdrup HU (1955) The place of physical oceanography in oceanographic research. J. Mar. Res. 14: 287. Tomczak M and Godfrey JS (1994) Regional Oceanography: An Introduction. Oxford: Pergamon.