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

Abstract

Planktonic gross primary production (GPP), community respiration (CR), and nitrification (NIT) were measured monthly in the Scheldt estuary by the oxygen incubation method in 2003. No significant evolution of planktonic GPP was observed since the 1990s with high rates in the freshwater area (salinity 0; 97 6 65 mmol C m 22 d 21 ) decreasing seaward (22-37 mmol C m 22 d 21 ). A significant decrease of NIT was observed with regard to previous investigations although this process still represents up to 20% of total organic matter production in the inner estuary. Planktonic CR was highest in the inner estuary and seemed to be mainly controlled by external organic matter inputs. Planktonic net community production was negative most of the time in the estuary with values ranging from 2300 to 165 mmol C m 22 d 21 . Whole estuary net ecosystem production (NEP) was investigated on an annual scale using the results mentioned above and published benthic metabolic rates. A NEP of 239 6 8 mmol C m 22 d 21 was estimated, which confirms the strong heterotrophic status of this highly nutrified estuary. NEP rates were computed from June to December 2003 to compare with results derived from the Land-Ocean Interaction in the Coastal Zone budgeting procedure applied to dissolved inorganic phosphorus and carbon (DIP and DIC). DIP budgets failed to provide realistic estimates in the inner estuary where abiotic processes account for more than 50% of the nonconservative DIP flux. DIC budgets predicted a much lower NEP than in situ incubations (2109 6 31 versus 242 6 9 mmol C m 22 d 21 ) although, as each approach is associated with several critical assumptions, the source of this discrepancy remains unclear.

Primary production during nutrient-induced blooms at elevated CO<sub>2</sub> concentrations

“…drawdown of DIC, have also been reported by others. In the Scheldt estuary, estimates of NCP in bottle incubation were 2-fold higher than those obtained from DIC budgets (Gazeau et al, 2005). In a net heterotrophic system, direct measurements of primary production and respiration were 4-fold higher than obtained from the geochemical approach (Wang et al, 2005).…”

The stoichiometry of inorganic carbon and nutrient removal in the Mississippi River plume and adjacent continental shelf

“…However, DIC and nutrient concentrations in plumes are also affected by temporally variable and spatially heterogeneous river-ocean mixing dynamics, which can complicate the interpretation of the biological signals (as opposed to those within spatially bounded estuaries, Gazeau et al, 2005). In addition, there are extensive intertidal wetlands along the Louisiana coast that are usually neglected in considerations of the Mississippi River loading, which could be significant sources of dissolved nutrients such as Si .…”

Photosynthetic characteristics of the phytoplankton in the Scheldt estuary: community and single-cell fluorescence measurements

“…Net primary production is possible only when the euphotic-mixing depth ratio remains sufficiently high (Cole et al, 1992;Soetaert et al, 1994;Kromkamp & Peene, 1995;Desmit et al, 2005). In addition, bacterial production and bacterial respiration are often high due to elevated organic carbon loading (Goosen et al, 1999;Gazeau et al, 2005). The combination of low primary production and high bacterial respiration means that most estuaries are net heterotrophic systems (Heip et al, 1995).…”