Phytoplankton: Relationships between phytoplankton, nutrients, oxygen flux and secondary producers

Abstract

In an intensive multi-component eight-year study (1974–1981) of mesohaline waters along the shallow western shore of Chesapeake Bay, phytoplankton species composition, productivity and pigment concentrations were routinely determined in monthly sample collections. In general, small, flagellated cells dominated total cell densities in the summer months while centric diatoms dominated assemblages in the winter, spring and fall. Highest phytoplankton densities, ≥53 × 106 cells·L−1, were observed in the spring bloom in bottom waters. Highest chlorophyll concentrations were observed during peaks of biomass-dominant cells, i.e., diatoms or dinoflagellates, rather than during periods of nanoflagellate dominance. Primary productivity was highest in the summer, ranging from 1.37 to 2.75 gC·m−2·d−1 and generally coincided with high dinoflagellate densities. Annual rates ranged from 167 to 392 gC·m−2. Nutrient concentrations and elemental ratios suggest occasional nutrient limitation of phytoplankton production in the region, with a limited phosphorus data set suggesting P-limitation in spring. Nitrogen limitation was possible in approximately one-half of the summer and early fall months sampled (13 of 24). However, regenerated nitrogen could supply rapid and frequent pulses of dissolved inorganic nitrogen throughout the summer and fall. A significant inverse relationship, observed between changes in silicate concentrations and fluctuations in diatom densities throughout the study period, suggests that silicate limitation may occasionally lead to the collapse of the spring bloom in mesohaline waters of Chesapeake Bay. Grazing by macrozooplankton (>73 μm) removed 17 to 83% of phytoplankton standing stock during the late summer (July–September) of each year with ≤15% removed by planktonic herbivores throughout the rest of the year. High salinities, densities, nutrient levels and declining oxygen concentrations in near-bottom waters at 11 m in the study area indicate that sub-pycnocline, mid-channel waters may frequently displace nearshore waters in the region.