The partition discussed here is of the upper ocean and rests principally on observed or inferred regional discontinuities in physical processes, particularly those that affect the stability of the upper kilometer of the ocean; regional differences in other ecologically significant variables, such as irradiance, are also considered. In short, the partition is based largely on the factors invoked by the Sverdrup model and proceeds from the suggestion made in Chapter 4 that six simple models, or cases, are sufficient to accommodate the observed range of pelagic production mechanisms. You will remember that I suggested that this might be more useful in partitioning oceanic phytoplankton ecology than the simpler analysis of Margalef, who defined only four quadrants in the nutrient/turbulence relationship.
Little rearrangement and only modest consolidation is required to apply these six cases to my earlier suggestions to recognize four biomes, or basic vegetation types, within the pelagial realm of the oceans; these are very similar in concept to those of Beklemishev (1969) and to the eco-regions of Bailey (1983). With each biome is associated one or more of the six models already discussed, as in the following arrangement:
Polar biome: where the mixed-layer depth is constrained by a surface brackish layer that forms each spring in the marginal ice zone:
Case 1—Polar irradiance-mediated production peak.
Westerlies biome: where the mixed-layer depth is forced largely by local winds and by local irradiance:
Case 2—Nutrient-limited spring production peak.
Case 3—Winter-spring production with nutrient limitation.
Trades biome: where the mixed-layer depth is forced by geostrophic adjustment on an ocean-basin scale to local or distant wind forcing:
Case 4—Small-amplitude response to trade wind seasonality.
Case 5—Large-amplitude response to monsoon reversal of trade winds.
Coastal biome: where many diverse coastal processes modify the mixed-layer depth and nutrient inputs:
Case 6—Intermittent production at coastal divergences and upwellings.
While reviewing the general properties and boundaries of these biomes it will be useful to bear in mind that only the trades biome represents a continuous body of water in each ocean. The polar and westerlies biomes each exist as two separated boreal and austral units and, furthermore, because land masses are not uniformly distributed in each hemisphere, their boreal and austral expressions have individual characteristics: thus, the boreal polar biome comprises a mediterranean sea containing a major archipelago, whereas the austral polar biome comprises an annular open ocean surrounding a central continent. Despite such asymmetries, the degree of ecological commonality is sufficient to support the biome concept presented again here.
In putting the case for this partition, I want to emphasize that the definition offered refers to an ideal ocean on a landless globe. In applying it to the real ocean, pragmatism must be used in setting limits to the individual partitions, this being especially true for the Coastal Biome, where the distribution of shallow continental shelves and deep basins may be too fractal for the dogmatic application of the definitions. Similarly, partially isolated basins such as the Mediterranean and Caribbean, or archipelagic regions such as the SW Pacific, also require to be treated pragmatically.
There is, obviously, a wider set of ecological factors that, if they were available and if we could identify discontinuities in their global fields, could be used to define more precisely the ecological characteristics and boundaries of each biome. For instance, it would be useful to quantify how plankton diversity (and hence the complexity of trophic
The Four Primary Biomes of the Upper Ocean 91
networks) changes regionally, because this is clearly an attribute likely to take characteristic values for individual ecosystems (or biomes): unfortunately, for very few water bodies is there a complete enumeration of the species present, let alone any comprehensive measure of characteristic ecosystem diversity. A good illustration of this problem is provided by one of the most complete and recent enumerations of pelagic biota, that of Margalef (1994), which compares species lists of phytoplankton from small flagellates to very large dinoflagellates and diatoms taken at two stations, one each in the Caribbean and in the western Mediterranean. These yielded lists of 353 and 257 named species, respectively, with about 50 more unidentified taxa at each. To complete an inventory of autotrophic cells, to these counts would have to be added the flagellate flora of the nano- and ultraplankton and the cells of the photosynthetic picoplankton (cyanobacteria and prochlorophytes). It is very unusual to be able to locate even single samples of the heterotrophs (micronekton, meso- and micro-metazoans, and the protists) that have been analyzed taxonomically as well as Margalef ’s phytoplankton.
To obtain regional fields of diversity within which we might hope to discern disconti- nuities, we would need observations such as those described by Margalef (but completed by inclusion of all taxonomic groups) on a suitable grid across the ocean, repeated sea- sonally. Obviously, as was discussed in Chapter 2, such information exists for no group of organisms, and probably for no location in all the oceans is there a complete listing of the kind required.
Another way of proceeding would be to seek information on the global distribution of biota at higher taxonomic levels than species, hoping to aggregate these into ecologically meaningful groups. It is reasonable to hope that such data might be accessible in a more uniform format than the almost hopelessly diverse descriptions of species lists at individual stations that can be found in the biogeographic literature. Fortunately, the Smithsonian Institution has for many years used a standardized first-order sorting technique for the plankton samples it archives, and these protocols have been adopted uniformly by other sorting centers. An archive of these sheets from several plankton sorting centers thus enables a first-order analysis of the composition of zooplankton globally; in one case, this was done with data from 4166 stations from all oceans where sampling nets had been worked between the surface and 250 m (Longhurst, 1985b).
The counts were allocated to six functional groups (gelatinous predators, raptorial predators, micro- and macroparticle herbivores, omnivores, and detritivores) and also among taxonomic groups (medusae, siphonophores, chaetognaths, polychaetes, ostra- cods, copepods, mysids, euphausiids, and pteropods). These counts were then stratified regionally and seasonally to represent first-order differences between oceans and conti- nental shelf faunas of the polar, temperate, and tropical zones. The general results are shown in Table 6.1, which enables the quantification of some latitudinal trends and seasonal changes in plankton composition that were well known for only a few study sites, although also suspected to occur more generally.
To illustrate the possibilities offered by such data, note how they quantify the change in dominance of copepods from 67% of zooplankton biomass in polar seas to 33% in the tropics, and how predators (both gelatinous and raptorial) increase from 22% in high latitudes to 47% at low latitudes. A striking aspect of these zonal differences is the increas- ing contribution of gelatinous predators equatorward: from 0.9% of total zooplankton biomass in polar latitudes to 14.3% in the tropics. The more equal distribution of relative biomass among both the taxonomic and trophic groups in the tropical, compared with the polar, seas is also clearly recorded.
Other data can also be consulted for confirmation of regional boundaries established otherwise: consider the case of calanoid diversity in the samples from the Continuous Plankton Recorder (CPR) survey of the North Atlantic. Long-term taxonomic richness of calanoid copepods in several thousand transects (Beaugrandet al., 2000a) shows a zonal
92Chapter6:Biomes:ThePrimaryPartition
Table 6.1. Relative composition of zooplankton in coastal and oceanic biomes derived from data on >4000 samples held in the Smithsonian and Kuroshio Plankton Sorting Centers. Gelatinous predators are medusae and siphonophores; raptorial predators are chaetognaths and some genera of cladocerans and copepods, with all amphipods, annelids, and gymnosomes;
macro-filtering herbivores are some genera of cladocerans, copepods, and thecosomatous mollusks; gelatinous herbivores are tunicates, appendiculaians, and doliolids with mucous-net filtration; omnivores are some genera of copepods, some euphausiids, all mysids, and penaeids; detritivores are all ostracods.
Medusaevar. Siphonophora Chaetognatha Polychaeta Cladocera Ostracoda Copepoda Amphipoda Mysidae Euphausiidae Penaeidae Pteropoda Appendiculariae Salpida Doliolida
N individuals (%)
OCEANIC Polar 0.06 0.15 3.88 0.23 0.00 2.45 86.32 0.56 0.00 0.52 0.02 2.44 3.32 0.02 0.00
Westerlies 1.39 0.45 3.99 0.54 0.47 3.12 80.45 0.81 0.02 2.77 0.33 1.76 2.79 0.52 0.12 Trades 0.85 2.61 6.38 0.85 0.14 3.39 72.31 0.54 0.15 3.02 0.30 1.70 3.85 2.10 0.66 COASTAL Westerlies 0.22 0.59 3.32 0.39 7.07 0.31 80.10 0.78 0.07 1.02 0.27 1.50 3.51 0.46 0.05 Trades 0.67 2.11 5.83 0.43 6.46 5.72 62.95 0.82 0.02 1.17 3.15 0.60 5.45 5.15 0.10 Carbon biomass (%)
OCEANIC Polar 0.23 0.75 8.62 0.47 0.00 3.05 67.58 4.57 0.00 7.66 0.13 6.73 0.18 0.03 0.00
Westerlies 3.28 1.78 8.62 0.47 0.00 3.05 67.58 4.57 0.00 7.66 0.13 6.73 0.18 0.03 0.00 Trades 1.98 8.51 9.00 1.10 0.01 2.61 33.13 2.58 0.00 30.21 5.02 2.66 0.10 2.89 0.01 COASTAL Westerlies 0.62 2.66 6.51 0.79 0.78 0.28 64.66 5.22 0.00 11.79 1.11 4.83 0.17 0.80 0.00 Trades 1.39 7.88 9.81 0.64 0.48 4.98 37.51 4.25 0.00 10.67 15.26 1.15 0.18 5.80 0.00
TheFourPrimaryBiomesoftheUpperOcean93 Carbon biomass (%) Entrapping
predators
Raptorial predators
Micrivorous herbivores
Macrivorous herbivores
Omnivores Detritivores
OCEANIC Polar 0.88 21.72 0.21 56.13 18.08 2.98
Westerlies 4.08 33.65 0.68 19.80 39.80 1.98
Trades 14.30 33.05 4.22 20.48 24.64 3.44
COASTAL Westerlies 3.62 14.01 1.07 43.08 37.90 0.31
Trades 9.84 28.14 6.28 7.84 42.58 5.31
Source: Longhurst, 1985.
94 Chapter 6: Biomes: The Primary Partition
discontinuity across the ocean at between 40 and 50N. Taxonomic richness, supported by five separate diversity indices, followed very closely, as the authors commented, the boundaries between the partition between biomes discussed here. The authors note that the poleward decrease of taxonomic diversity anticipated in the pelagic ecosystem is discontinuous, and that the gradient is interrupted by local variability where different surface water bodies are interleaved.
Such information, although very restricted in scope, does gives some confidence that the primary partition discussed here is based on reality.