Primary production by suspended and benthic microalgae in a turbid estuary: Time-scales of variability in San Antonio Bay, Texas

Abstract

The within-day, between-day and month-to-month variability of light attenuation and microalgal chlorophyll a (chl a) and photosynthetic response was measured in San Antonio Bay, Texas, USA. Waters were shallow (<2 m) and turbid (attenuation coefficients of 0.7 to 15.3 m(-1), yet daily rates of primary production (0.1 to 2.5 g C m(-2) d(-1)) were comparable to those in much deeper and clearer estuaries. Chl a concentrations in the sediment (459 to 7837 mg m(-3) in the surficial millimeter) were much higher than those in the water column (4 to 48 mg m(-3)). The benthic assemblages were photosynthetically competent, but daily benthic primary productivity was low (0.00 to 0.09 g C m(-2) d(-1); an average of 2 % of productivity in the water column) because of the very low irradiances at the sediment-water interface. The high rates of production by the suspended microalgae were largely due to high chlorophyll-specific light-saturated rates of photosynthesis, P-m(chl) [3.0 to 24.4 g C (g chl a)(-1) h(-1)], which were correlated positively with temperature and inversely with the mean irradiance in the water column. The between-day and temperature-independent variation in P-m(chl) was also correlated with F(v)Chl(-1), an index of the proportion of functional photosystem II reaction centers. In turn, within-day variability in F(v)Chl(-1) was inversely correlated with the mean irradiance in the water column in 8 of 10 observations, a pattern consistent with photoinhibition. A depression of P-m(chl) caused by photoinhibition throughout the water column may therefore be responsible for the inverse trend of P-m(chl) with mean irradiance in the water column. Short-term (h) variability in suspended chl a and turbidity was high (coefficient of variation = 13 to 75 %), but estimates of daily productivity could be predicted with reasonable fidelity (mean error 27 %) from a single midday determination of chl a, the photosynthesis versus irradiance response and the attenuation coefficient, along with daily incident radiation. The predictive power of a single observation was due to coherence in the variation of chl a, P-m(chl) and the attenuation coefficient in the water column: the decrease in mean irradiance caused by resuspension was compensated for by concomitant increases in P-m(chl) and suspended chl a. Between-day variability in productivity of 15 to 52% approached month-to-month differences, so the optimal use of resources in monitoring productivity would be to take single samples daily.