Research Product

Phytoplankton nutrient limitation was studied in a sub-estuary of lower Pensacola Bay using several techniques. 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 growth more than single nutrients, indicating incipient co-limitation. Variability of in-situ alkaline phosphatase (Pase) activity was not clearly related to the inorganic nutrient concentrations or nutrient ratios. However, changes in Pase activity after nutrient additions were consistent within the bioassays. Despite evidence for nutrient limitation, the maximum quantum yield of photosystem II photochemistry (Fv/Fm) was generally high, indicating that nutrient limitation was not severe enough to reduce photosynthetic efficiency. Techniques like Pase activity and Fv/Fm measure the physiological state of the in situ phytoplankton community, while nutrient-addition bioassays test whether community production and yield could change with additional nutrient inputs. This distinction explains why phytoplankton in this environment evidenced nutrient limitation without apparent physiological impairment. From this perspective, the results indicate that Pensacola Bay would be very sensitive to increased N and P loads, causing strongly increased phytoplankton production and yield (i.e. eutrophication) with potentially negative ecosystem consequences.