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PHYTOPLANKTON NUTRIENT STATUS AND VARIABLE FLUORESCENCE MEASUREMENTS IN A GULF COAST ESTUARY
Citation:
Juhl, A R. AND M C. Murrell. PHYTOPLANKTON NUTRIENT STATUS AND VARIABLE FLUORESCENCE MEASUREMENTS IN A GULF COAST ESTUARY. Presented at ASLO/AGU Ocean Sciences Meeting, Honolulu, HI, Feb. 11-15, 2002.
Description:
Changes in variable fluorescence parameters such as the maximum quantum yield of fluorescence (a.k.a. photosynthetic efficiency and Fv/Fm) have been related to nutrient status in single-species cultures. To test if changes in Fv/Fm of mixed natural assemblages were related to nutrient status, variable fluorescence parameters were measured during nutrient-addition bioassays conducted on samples collected throughout 2001 in Santa Rosa Sound. Santa Rosa Sound is a component of the Pensacola Bay system located along the
northern Gulf of Mexico (Florida, USA). 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 nutrient addition generally increased Fv/Fm. One way to
interpret increased Fv/Fm would be that nutrient addition enhanced physiological condition of the entire community, leading to faster growth. However, our observations suggest that changes in Fv/Fm were related to
changes in community composition. 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/Fn 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.