ANNUAL CYCLE AND INTERANNUAL VARIABILITY OF ECOSYSTEM METABOLISM IN ATEMPERATE CLIMATE EMBAYMENT

We have studied the net and gross metabolism of Tomales Bay, a tempera te climate estuary in northern California. Tomales Bay has proved to b e heterotrophic, implying that the bay oxidizes a subsidy of organic c arbon from outside the system, in excess of inorganic nutrients suppli ed to it from outside and in addition to material cycling within it. N et organic oxidation releases dissolved inorganic nutrients, and the s ystem exports these dissolved inorganic products. Dissolved inorganic phosphorus is exported to the ocean via mixing and constitutes the mos t direct record of net ecosystem production (NEP). Excess dissolved in organic nitrogen is lost to denitrification. Excess dissolved inorgani c carbon largely results in alkalinity elevation and hydrographic expo rt of alkalinity due to sulfate reduction. The negative NEP of this sy stem results in little release of CO2 to the atmosphere, because of th is alkalinity elevation. A major purpose of the study was to ascertain the relative importance of various sources of organic material suppli ed to the system from outside its boundaries and undergoing net reacti ons within it. In order to address the question, we used stoichiometri cally linked whole-system budgets of carbon, nitrogen, and phosphorus. The difference between dissolved inorganic phosphorus (DIP) fluxes to and from the bay is a measure of net internal sources or sinks of DIP and is used as a quantitative index of NEP, with the assumption that the C:P ratio of organic matter is constant (similar to 106:1). The sy stem is thus defined in terms of water column dissolved material compo sition; this definition includes time, as well as space. Net changes i n the standing stocks of dissolved materials can originate from (spati al) transport to or from the system or from internal (temporal) transf ormations between the dissolved and particulate materials (i.e., chang es in organic storage). Over the 8-yr study, the system respired 12 mm ol.m(-2).d(-1) more organic C than the internal system primary product ion of similar to 100 mmol.m(-2)d(-1). The system is thus heterotrophi c by similar to 10%, with substantial seasonality in the extent of het erotrophy. By deconvoluting the time series of NEP into a seasonal cyc le and interannual variation, we infer that terrestrial and marine sou rces each account for about half of the carbon required to support neg ative NEP in this system, but with quite different turnover time scale s. Temporal response of NEP to terrigenous input appears to be extreme ly modulated, so that there is no obvious immediate (same year) respon se to extreme interannual variation in terrigenous organic loading. In contrast, NEP responds both interannually and seasonally to marine or ganic inputs. We interpret the differences in response to loading of t errestrial vs. marine organic matter as reflecting differences in the reactivity of these carbon reservoirs.