Seasonal Impacts of Food Resources in the Marine Environment
Primary productivity is a primarily function of sunlight an available nutrients. Figure 14.20 shows the primary productivity of the three zones: tropical, temperate, and polar.
Primary productivity in the tropical zone is limited not by sunlight it receives, but because a thick thermocline prevents nutrients from moving up in to the surface in the photic zone. Primary productivity in the polar zones are most intense in the summer months when both sunlight and nutrients are available.
Primary productivity in the temperate zones have two peaks in the spring and fall. Productivity is limited in the summer months because a thermocline builds up, shutting down the nutrient supply to the upper ocean. Primary productivity increases in the spring when sunlight increases and before a strong thermocline shuts down the supply of nutrients. Productivity also increases in the fall when cooler weather breaks up thermocline (allowing upwelling of nutrients) while ample sunlight is still available to support phytoplankton growth.
Impacts on Consumers
Primary, secondary, and tertiary consumers follow the cycles of primary productivity described above. A proliferation of zooplankton (primary consumers) occurs when their food (phytoplankton) becomes increasingly available. Zooplankton populations grow at the ultimate expense of the phytoplankton, and their populations peak, phytoplankton first, then zooplankton next. Secondary and tertiary consumer populations consume these recourses, and then migrate to search of other sources of food following the blooms in productivity northward in the spring and then returning south for the winter.
Figure 14.21 compares the biomass of phytoplankton and zooplankton for the tropical, temperate, and polar zones through the months of year. Figure 14.22 compares the availability of nutrients and sunlight with biomass of phytoplankton and zooplankton for the temperate zone through seasons of the year.
|Figure 14.20. Primary productivity in 3 zones.
|Figure 14.21. Plankton biomass by season in three zones.
|Figure 14.22. Temperate zone productivity by seasons.