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OBSERVATIONS OF PHYTOPLANKTON SIZE STRUCTURE, NUTRIENTS, VARIABLE FLOURESCENCE, AND ALGAL PHOSPHATASE ACTIVITY IN A GULF COAST ESTUARY
Citation:
Murrell, M C. AND A R. Juhl. OBSERVATIONS OF PHYTOPLANKTON SIZE STRUCTURE, NUTRIENTS, VARIABLE FLOURESCENCE, AND ALGAL PHOSPHATASE ACTIVITY IN A GULF COAST ESTUARY. Presented at ASL Conference, Honolulu, HI, Feb. 10-14, 2002.
Description:
During 2001, phytoplankton dynamics, physiology, and related environmental conditions were studied in Santa Rosa Sound, Florida, USA, at near-weekly intervals. Santa Rosa Sound is a component of the Pensacola Bay system located in the northern Gulf of Mexico. Environmental parameters measured included temperature, salinity, inorganic nutrients, and size-fractionated chlorophyll. Phytoplankton variable fluorescence parameters were measured with a fast-repetition-rate-fluorometer (FRRF) to evaluate its potential as a tool for assessing in
situ phytoplankton nutrient status. Algal phosphatase (Pase) activities were used to examine relative phosphorus stress. The time series data showed that temperature varied in a typical seasonal fashion and that salinity varied in response to rainfall events. Dissolved inorganic nitrogen (DIN, mostly as NH4) concentrations ranged from < 0.5 to 5.2 ?M, dissolved inorganic phosphorus (DIP) concentrations ranged from < 0.03 to O.3 ?M, and dissolved silica (DSi) concentrations ranged from 6 to 32 ?M. DIN and DSi (but not DIP) were negatively
correlated with salinity suggesting that rainfall and local runoff were the primary sources. Chlorophyll concentrations peaked during spring (10-15 ?g1-1), then dropped by ca. 50% and remained uniformly low through summer and fall. Phytoplankton size structure typically was evenly distributed between large (> 5 ?m) and small (< 5 ?m) cell fractions, except in later summer/fall when small cells often dominated. The maximum quantum yield of chlorophyll fluorescence (Fv/Fm) for the bulk community varied from 0.54 to 0.61 with lower values during spring and higher values during summer. The large size fraction consistently had higher Fv/Fm
values, although the difference was usually small. The generally high values of bulk Fv/Fm indicated that the phytoplankton community was not nutrient starved in situ. However, a companion study found that nutrient additions always stimulated phytoplankton growth, suggesting that at least a portion of the community was nutrient limited. Although not statistically significant, Fv/Fm was more strongly related to DIP than DIN concentration. Chlorophyll-normalized Pase activity was positively correlated (P < 0.05) with DIN concentration
(and negatively with salinity), suggesting that the supply of DIN was driving phytoplankton towards P stress.