Nutrient limitation of phytoplankton growth and physiology in a subtropical estuary (Pensacola Bay, Florida)

Phytoplankton nutrient limitation was studied in a sub-estuary of lower Pensacola Bay using multiple techniques in parallel. Nutrient-addition bioassays indicated year-round nutrient limitation, in contrast to seasonal patterns often described for higher-latitude estuaries. Although an earlier study found frequent P-limitation in Pensacola Bay, N-limitation dominated during this study, despite dissolved inorganic N:P ratios consistently above Redfield proportions. However, combined N and P additions enhanced phytoplankton growth more than single nutrients, indicating incipient co-limitation. In-situ alkaline phosphatase (Pase) activity was not clearly related to inorganic nutrient concentrations or ratios. However, changes in Pase activity after nutrient additions were consistent with the other bioassay data. Despite evidence for nutrient limitation from the bioassays, the maximum quantum yield of photosystem II photochemistry (F-v/F-m) was generally high, indicating that nutrient limitation was not severe enough to reduce photosynthetic efficiency. Techniques like Pase activity and F-v/F-m measure the physiological state of the in-situ phytoplankton community, while nutrient-addition bioassays test whether community production and yield would change with additional nutrient inputs. This distinction explains how the phytoplankton community in this environment could be nutrient limited without apparent physiological impairment. Physiological measurements may be less sensitive than nutrient-addition bioassays in diagnosing certain aspects of phytoplankton nutrient limitation. The results indicate that Pensacola Bay would be sensitive to elevated N and P loadings, increasing phytoplankton production and yield with potentially negative ecosystem consequences.