Organic carbon balance and net ecosystem metabolism in Chesapeake Bay

Abstract

The major fluxes of organic carbon associated with physical transport and biological metabolism were compiled, analyzed and compared for the mainstem portion of Chesapeake Bay (USA). In addition, 5 independent methods were used to calculate the annual mean net ecosystem metabolism (NEM = production -respiration) for the integrated Bay. These methods, which employed biogeochemical models, nutrient mass-balances and summat~on of individual organic carbon fluxes, yielded remarkably similar estimates, with a mean NEM of +50 g C m-2 yr.' (i SE = 7.51, which is approximately 8 % of the estimated annual average gross primary production. These calculat~ons suggest a strong cross-sectional pattern in NEM throughout the Bay, wherein net heterotrophic metabolism prevails in the pelagic zones of the rnaln channel, while net autotrophy occurs in the littoral zones which flank the deeper central area. For computational purposes, the estuary was separated ~n t o 3 regions along the land-sea gradient: (1) the oligohaline Upper Bay (1 1 "L of total area); (2) the mesohaline Wd Bay (36% of area); and (3) the polyhallne Lower Bay (53% of area). A distinct regional trend in NEM was observed along this salinity gradient, with net heterotrophy (NEM =-87 g C m-' yr-') in the Upper Bay, balanced metabolism in the Mid Bay and net autotrophy (NEM = +92 g C m-' y r ' ) in the Lower Bay. As a consequence of overall net autotrophy, the ratio of dissolved inorganic nitrogen (DIN) to total organic nitrogen (TON) changed from D1N:TON = 5.1 for riverine inputs to D1N:TON = 0.04 for water exported to the ocean. A striking feature of this organic C mass-balance was the relative dominance of biologically mediated metabolic fluxes compared to physical transport fluxes. The overall ratio of physical TOC inputs (I) to biotic primary production (P) was 0.08 for the whole estuary, but varied dramatically from 2.3 in the Upper Bay to 0.03 in the Mid and Lower Bay regions. Similarly. ecosystem respiration was some 6-fold higher than the sum of all physical carbon sinks. This general negative correspondence between 1:P ratio and NEM, which occurred among Bay regions, was also evident in data available for organic C fluxes in other coastal ecosystems. An inverse relationship between NEM and P, postulated in a previous study, did not apply to Chesapeake Bay, and closer examination of available data revealed the importance of the loading ratio of DIN:TOC as a key control on coastal NEM. It is proposed here that the general global trend of coastal eutrophicatlon will lead to increasing values of NEM in estuaries worldwide. The management implications of this trend are complex, involv~ng both increased potential fisheries harvest and decreased demersal habitat. K E Y WORDS: Net ecosystem metabolism Production . Respiration Organic carbon . Inorganic nutrients Estuaries. Chesapeake Bay INTRODUCTIONRates of organic production in estuaries and other coastal ecosystems are among the highest in the bio-

Planktonic and whole system metabolism in a nutrient-rich estuary (the Scheldt estuary)

“…Caffrey's O 2 based rates were converted to carbon units using the same O 2 to carbon conversion factors (PQ and RQ); the validity of these factors and the discrepancy between incubation and open-system approaches will be discussed in detail below. Kemp et al (1997) found a significant positive relationship between NEP estimates in several systems and the ratio between dissolved inorganic nitrogen and organic matter loadings. The NEP computed in the Scheldt estuary based on in situ incubations does not follow this pattern with a low NEP and high inputs for both nutrient and organic matter.…”

“…Linking land to the ocean, they are often greatly influenced by human activities, including enhanced organic matter and nutrient loadings. Among other parameters, the balance between organic matter and nutrient loading is critical in determining the balance between autotrophy and heterotrophy at the ecosystem level (Kemp et al 1997;Eyre and McKee 2002). …”

“…As mentioned above, several techniques are available to attain such an objective but very few studies have focused on comparing different methods (Kemp et al 1997;Gazeau et al 2005) and testing their validity in a variety of ecosystem types.…”

The Effects of System-Level Nutrient Enrichment on Bacterioplankton Production in a Tidally-Influenced Estuary**

“…This pattern may also be driven by lower growth efficiencies at intermediate salinities that result from elevated rates of bacterial respiration and reduced BP and are related to shifts in bacterioplankton phylogeny and/or metabolism associated with changing salinity (BOUVIER and DEL GIOR- GIO, 2002;CRUMP, AMBRUST, and BAROSS, 1999;DEL GIOR-GIO and BOUVIER, 2002;YOKOKAWA et al, 2004). In Chesapeake Bay, the apparent negative effect of salinity on the coherence of BP and CHLA may be attributed to the welldocumented shift from net heterotrophy in the upper bay to higher phytoplankton biomass and net autotrophy in the lower bay (KEMP et al, 1997). Shifts of this nature in the relationship between BP and CHLA appear to represent a recurring property along salinity gradients across a wide range of estuarine ecosystems, although the mechanisms are not well understood and may differ among estuaries.…”

Size structure and the production/respiration balance in a coastal plankton community

“…Finally, in an attempt to deduce processes governing these relationships, we examine temporal and spatial variability in the partitioning of rates and P : R ratios among the two size fractions. This study was conducted in the surface waters of the mesohaline to polyhaline region of Chesapeake Bay, which is characterized by a wide range of P and R rates in the plankton community (Smith and Kemp 1995) and by high overall net autotrophy (Kemp et al 1997). …”