CHANGES IN PRODUCTION AND RESPIRATION DURING A SPRING PHYTOPLANKTON BLOOM IN SAN-FRANCISCO BAY, CALIFORNIA, USA - IMPLICATIONS FOR NET ECOSYSTEM METABOLISM

We present results of an intensive sampling program designed to measur e weekly changes in ecosystem respiration (oxygen consumption in the w ater column and sediments) around the 1996 spring bloom in South San F rancisco Bay, California, USA. Measurements were made at a shallow sit e (2 m, where mean photic depth was 60% of the water column height) an d a deep site (15 m, mean photic depth was only 20% of the water colum n). We also estimated phytoplankton primary production weekly at both sites to develop estimates of net oxygen flux as the sum of pelagic pr oduction (PP), pelagic respiration (PR) and benthic respiration (BR). Over the 14 wk period from February 5 to May 14, PP ranged from 2 to 2 10, PR from 9 to 289, and BR from 0.1 to 48 mmol O-2 m(-2) d(-1), illu strating large variability of estuarine oxygen fluxes at the weekly ti me scale. Pelagic production exceeded total respiration at the shallow site, but not at the deep site, demonstrating that the shallow domain s are net autotrophic but the deep domains are net heterotrophic, even during the period of the spring bloom. If we take into account the po tential primary production by benthic microalgae, the estuary as a who le is net autotrophic during spring, net heterotrophic during the nonb loom seasons, and has a balanced net metabolism over a full annual per iod. The seasonal shift from net autotrophy to heterotrophy during the transition from spring to summer was accompanied by a large shift fro m dominance by pelagic respiration to dominance by benthic respiration . This suggests that changes in net ecosystem metabolism can reflect c hanges in the pathways of energy flow in shallow coastal ecosystems.