Ll

(activity + basal metabolism)

Production

(growth + reproduction)

Unutilized

(waste)

and adaptations reflect both the food availability and energy allocation among species and genders at different stages of their life histories.

Trophic dynamics across the Earth system, in response to sunlight (Fig. 7.1) and nutrient availability as limiting factors, can be illustrated by the cyclic production of phytoplankton and herbivorous zooplankton (Fig. 9.11). At low latitudes, where nutrient levels are low and solar radiation is constantly high throughout the year, there are relatively low-amplitude changes in the production of the primary producers and their consumers. In the mid-latitudes, taking advantage of winter nutrient accumulations, phytoplankton populations bloom with the return of spring. Subsequently, thriving zooplankton populations appear until they have depleted their food, at which point the phytoplankton can once again resume their production through winter onset.

FIGURE 9.11 Generalized seasonal relationships between phytoplankton blooms and their subsequent consumption by herbivorous zooplankton—illustrating basic trophic dynamics (Fig. 9.7) across the Earth system. In the low latitudes, there is minimal seasonality in solar illumination (Fig. 7.1), temperature, and nutrient availability—all of which influence primary production. In the mid-latitudes, there are spring and fall phytoplankton blooms associated with solar variability as well relatively high nutrient levels. In the high latitudes, photosynthesis can occur only during the summer. Bloom amplitudes as well as the lag periods (horizontal bar) between primary and secondary production peaks decrease toward the lower latitudes. Adapted from Parsons and Takahashi (1973).

FIGURE 9.11 Generalized seasonal relationships between phytoplankton blooms and their subsequent consumption by herbivorous zooplankton—illustrating basic trophic dynamics (Fig. 9.7) across the Earth system. In the low latitudes, there is minimal seasonality in solar illumination (Fig. 7.1), temperature, and nutrient availability—all of which influence primary production. In the mid-latitudes, there are spring and fall phytoplankton blooms associated with solar variability as well relatively high nutrient levels. In the high latitudes, photosynthesis can occur only during the summer. Bloom amplitudes as well as the lag periods (horizontal bar) between primary and secondary production peaks decrease toward the lower latitudes. Adapted from Parsons and Takahashi (1973).

With the short pulse of summer sunlight in the high latitudes, like the seasonality of the region, there are bursts of phytoplankton and zooplankton production. Particularly in the Antarctic marine ecosystem, along with the advance and retreat of the sea ice (Fig. 8.1), there is a cascade of biological responses as herbivores up the trophic pyramid redirect their feeding strategies and energetics as photosynthesis turns completely on or off with the seasons (Figs. 9.3-9.10).

In addition to production cycles, trophic relationships [Fig. 9.7; Eq. (9.3)] provide a framework for interpreting the distribution of biomass throughout the world ocean. In general, there are three types of oceanic provinces that can be distinguished in relation to their underlying levels of primary production (Table 9.4). Oceanic areas, which cover the vast majority of the Earth's surface, generally are oligotrophic with exceedingly low nutrient concentrations for primary production. As a consequence, in these oceanic areas, many species are competing for limited energy that is inefficiently transferred across many trophic levels. Coastal marine habitats, which incorporate nutrients from terrestrial runoff, have higher levels of primary production with more efficient energy transfers among fewer trophic levels. However, the most productive marine habitats occur in upwelling areas where there are high nutrient concentrations for primary producers to support short efficient trophic pyramids.

Table 9.4 is particularly relevant because it reflects the nutritional sources from the sea that are utilized by humans. Based on the trophic level of the fish (which represents the food eaten directly by humans) and the ecological efficiency [Eq. (9.2)], it can be seen that less than 1% of the fish food from the sea is obtained from the open ocean—which accounts for 90% of the ocean area. In contrast, nearly half of the fish production comes from coastal areas. The amazing conclusion from Table 9.4 is that upwelling areas, which occupy less than one-tenth of

T ABLE 9.4 Organic Production in Oceanic Provinces a

Oceanic provinces

T ABLE 9.4 Organic Production in Oceanic Provinces a

Oceanic provinces

Ecosystem characteristics

Oceanic

Coastal

Upwelling

Percentage of ocean

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