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PHYTOPLANKTON DYNAMICS IN A GULF OF MEXICO ESTUARY: THE POTENTIAL USE OF PHOTO-PHYSIOLOGY AND ALGAL PHOSPHATASE ACTIVITY TO PREDICT NUTRIENT STATUS.
Citation:
Juhl, A. R. AND M C. Murrell. PHYTOPLANKTON DYNAMICS IN A GULF OF MEXICO ESTUARY: THE POTENTIAL USE OF PHOTO-PHYSIOLOGY AND ALGAL PHOSPHATASE ACTIVITY TO PREDICT NUTRIENT STATUS. Presented at Gulf Estuarine Research Society, New Orleans, LA, May 15-17, 2002.
Description:
Development of rapid techniques to determine in situ phytoplankton nutrient status could facilitate understanding of phytoplankton growth and species succession. Variable fluorescence parameters of phytoplankton communities can be easily and rapidly measured, and changes in parameters such as the maximum quantum yield of fluorescence (Fv/Fm) have been related to nutrient status in single-species cultures. To test if changes in Fv/Fm are useful in assessing nutrient status of mixed natural assemblages, variable fluorescence parameters were measured during nutrient-addition bioassays. Assays were conducted on
samples collected during 2001 in Santa Rosa Sound, a component of the Pensacola Bay estuarine system, located along the northern Gulf of Mexico (Florida, USA). During the study, nutrient additions always stimulated phytoplankton net growth. Nitrogen appeared to be the primary limiting nutrient in summer and fall, phosphorus was sometimes limiting in winter and spring. Combined N and P addition generally had a greater stimulatory effect than either nutrient alone. Trace metal or iron additions never had a stimulatory effect, either alone or in
combination with other nutrients. Initial values of Fv/Fm were frequently high (>0.55) but generally increased after addition of the primary limiting nutrient. One might interpret increased Fv/Fm and high growth following nutrient addition to indicate enhanced physiological condition of the entire community. However, our observations suggest that changes in Fv/Fm were related to changes in community composition. Variable fluorescence measurements on different size fractions suggest that nutrient addition had relatively little effect on large portions of the phytoplankton but promoted overgrowth of the community by large cells with high Fv/Fm
values. Large increases in Fv/Fm following nutrient addition coincided with large shifts in community size structure. When size structure changed little, nutrient additions had little effect on Fv/Fm, despite increased net growth. Interpreting variable fluorescence measurements within a mixed community can be complicated by such shifts in community composition.