Benthic biogeochemical cycling, nutrient stoichiometry, and carbon and nitrogen mass balances in a eutrophic freshwater bay

Abstract

Green Bay, while representing only ,7% of the surface area and ,1.4% of the volume of Lake Michigan, contains one-third of the watershed of the lake, and receives approximately one-third of the total nutrient loading to the Lake Michigan basin, largely from the Fox River at the southern end of the bay. With a history of eutrophic conditions dating back nearly a century, the southern portion of the bay behaves as an efficient nutrient and sediment trap, sequestering much of the annual carbon and nitrogen input within sediments accumulating at up to 1 cm per year. Depositional fluxes of organic matter varied from ,0.1 mol C m 22 yr 21 to .10 mol C m 22 yr 21 and were both fairly uniform in stoichiometric composition and relatively labile. Estimates of benthic recycling derived from pore-water concentration gradients, whole-sediment incubation experiments, and deposition-burial models of early diagenesis yielded an estimated 40% of the carbon and 50% of the nitrogen recycled back into the overlying water. Remineralization was relatively rapid with ,50% of the carbon remineralized within ,15 yr of deposition, and a mean residence time for metabolizable carbon and nitrogen in the sediments of 20 yr. On average, organic carbon regeneration occurred as 75% CO 2 , 15% CH 4 , and 10% dissolved organic carbon (DOC). Carbon and nitrogen budgets for the southern bay were based upon direct measurements of inputs and burial and upon estimates of export and production derived stoichiometrically from a coupled phosphorus budget. Loadings of organic carbon from rivers were ,3.7 mol m 22 yr 21 , 80% in the form of DOC and 20% as particulate organic carbon. These inputs were lost through export to northern Green Bay and Lake Michigan (39%), through sediment burial (26%), and net CO 2 release to the atmosphere (35%). Total carbon input, including new production, was 4.54 mol m 22 C yr 21 , equivalent to ,10% of the gross annual primary production. Nitrogen budget terms were less well quantified, with nitrogen export ,54% of total inputs and burial ,24%, leaving an unquantified residual loss term in the nitrogen budget of ,22%.

Physical and biogeochemical mechanisms of internal carbon cycling in Lake Michigan

“…[], however, do infer a decline in primary productivity prior to the arrival of Dreissena mussels based on increases in silica concentration. Furthermore, the impact of riverine inputs is most likely the greatest in Green Bay, which receives up to one‐third of the total nutrient loading to Lake Michigan, despite accounting for only 1.4% of total volume [ Klump et al ., ]. These substantial nutrient inputs combined with shallow depths and short residence times (<1 year) create a hypereutrophic system that is dramatically different from the rest of Lake Michigan and is not the target for this modeling study.…”

“…Carbon and phosphorus are traced based on a constant C:P of 200, which is within the range of 150–250 observed in phytoplankton in Lake Michigan [ Klump et al ., ; Bootsma et al ., ]. A constant C:P value is a necessary simplification that is commonly used in ocean biogeochemistry modeling studies [ Moore et al ., ; Dutkiewicz et al ., ] due to the lack of a mechanistic understanding for the observed variance.…”

Bottom‐up effects of lake sediment on pelagic food‐web compartments: a mesocosm study

“…This latter has been largely used to provide information on the degradation state and the biodegradability of the sediment, due to the large range of lability covered by the different components of OM (Meckler et al, 2004;Schubert et al, 2005). During its degradation, the sediment can release into the water column nutrients and dissolved organic matter (DOM; Reynolds, 1996;Klump, Fitzgerald & Waples, 2009), which will support both primary and bacterial production (Del Giorgio & Cole, 1998). As phytoplankton and bacteria are the basal resources for aquatic food webs, release of nutrients and OM from sediment might impact on food-web communities via a bottom-up forcing.…”

Carbon dioxide emissions and sediment organic carbon burials across a gradient of trophic state in eleven New Zealand lakes

“…5A), only a small proportion of the deposited carbon is permanently buried. The remainder is remineralized, producing dissolved organic and inorganic carbon as well as CO2 and CH4 to re-enter water column in which this process can be associated with atmospheric carbon emission (Klump et al, 2009). Thus, in this study, except for the geothermally influenced lakes, carbon mineralization rate was of the same magnitude as CO2 atmospheric emissions (Fig.…”

“…Sediment carbon deposition and burial were calculated based on a first-order diagenesis model for labile organic matter (Berner, 1980;Klump et al, 2009). The composition of OC deposited on the sediment surface was assumed to initially consist of two fractions: permanently buried and metabolizable material.…”