Trophic interactions and direct physical effects control phytoplankton biomass and production in an estuary

Abstract

The recent invasion of San Francisco Bay by the suspension-feeding clam Putamocorbula amurensis has provided an opportunity to document the ecological consequences of a major biological disturbance. Previous work over the last two decades has shown that phytoplankton biomass in the upper estuary is low (2-3 mg Chl a m-3) during seasonal periods of high river flow and short residence time, and it is usually high (peak > 30 mg Chl a m-3) during the summer-autumn seasons of low river flow and long residence time. However since P. amurensis became widespread and abundant in 1987, the summer phytoplankton biomass maximum has disappeared, presumably because of increased grazing pressure by this newly introduced species. For the period 1977-l 990, mean estimated primary production was only 39 g C mm2 yr-' during years when bivalve suspension feeders were abundant (>2,000 mm2), compared to 106 g C m-2 yr-' when bivalves were absent or present in low numbers. These observations support the hypothesis that seasonal and interannual fluctuations in estuarine phytoplankton biomass and primary production can be regulated jointly by direct physical effects (e.g. river-driven transport) and trophic interactions (episodes of enhanced grazing pressure by immigrant populations of benthic suspension feeders).

Reevaluating the Generality of an Empirical Model for Light-Limited Primary Production in the San Francisco Estuary

“…During 2006, incubations were terminated after 6 h and during 2007 incubations were maintained for 24 h, in part to account more fully for dark respiration and so more closely approximate net primary production (Eppley and Sharp 1975). The 24-h incubation period was also consistent with the majority of previous studies of primary production in the SFE (e.g., Cole and Cloern 1984;Alpine and Cloern 1992) making a direct comparison easier to interpret. Daily (24 h) estimates of primary production were calculated for 2006 by multiplying hourly uptake rates by day length omitting 1.5 h at dawn and dusk (the remaining day length is when 98 % of the solar irradiance strikes the Earth) and making no additional correction for dark respiration .…”

“…Like many other urbanized estuaries around the world, the SFE has experienced perturbations in key ecosystem drivers, including sediment and nutrient loads (Schoellhamer 2011;Jassby 2008), quantity and diversity of toxic pollutants (Brooks et al 2012), decreased phytoplankton biomass, and a loss of diatoms (e.g., Alpine and Cloern 1992;Kimmerer 2004;Dugdale et al 2007), and changes in zooplankton and fish (Mueller-Solger et al 2002;Kimmerer 2005). Although the original authors of the light-utilization model for the SFE (Cole and Cloern 1984) predicted that the empirical efficiency factor y would likely respond to such changes, this parameter has been previously recalculated for the northern estuary only twice (Alpine and Cloern 1992;Jassby et al 2002).…”

“…At the time of the original model calibration, Suisun Bay, in the northern SFE, was characterized by low phytoplankton growth rates but high (~60 μg chlorophyll L −1 ) summer phytoplankton biomass dominated by diatoms (Cloern 1979;Cole and Cloern 1984). In 1987 the invasive clam Corbula amurensis spread throughout the northern estuary, with grazing rates estimated to exceed phytoplankton growth (Alpine and Cloern 1992). Clam grazing is thought to have driven an abrupt decline in chlorophyll (now typically~3 to 4 μg L −1…”

“…The "light-utilization" productivity model (Harding et al 2002), developed during the 1980s for the light-limited San Francisco Estuary (SFE), provides estimates of depth-integrated primary production from measurements of chlorophyll, daily photosynthetically active radiation (PAR), and water column light attenuation (Cole and Cloern 1984). The original SFE model calibration was subsequently shown to be generally applicable to several light-limited estuaries (Cole and Cloern 1987) and invariant with time in the SFE (Alpine and Cloern 1992;Jassby et al 2002). As a result, the model is routinely applied using data from water quality monitoring as a means of evaluating changes to the primary production cycle in the SFE and for comparison with other estuaries (Cole and Cloern 1987;Kimmerer 2005).…”

The annual cycles of phytoplankton biomass

“…Cultural eutrophication or reduction of nutrient loading can cause massive change in phytoplankton dynamics (Anneville et al 2005;Jeppesen et al 2005) and modify the temporal succession and amplitude of phytoplankton fluctuations (Sommer et al 1993;Verity & Borkman 2010). Similarly, disruption of food-webs by introduced species can profoundly alter phytoplankton biomass (Alpine & Cloern 1992;Fahnenstiel et al 1995;Makarewicz et al 1999). Removal of top predators by fishing or introduction of non-indigenous species can also affect phytoplankton dynamics through trophic cascades (Frank et al 2005;Casini et al 2009).…”