Benthic primary productivity on the Georgia midcontinental shelf: Benthic flux measurements and high‐resolution, continuous in situ PAR records

Abstract

[1] Continuous, 7 year records of above-water, near-surface water column and nearbottom measurements of photosynthetically active radiation (PAR) were used in conjunction with in situ chamber measurements to estimate magnitude and seasonal variations in benthic primary productivity (BPP) on the Georgia continental shelf. Correlation between BPP and PAR flux was established with 19 light/dark benthic chamber and PAR flux measurements at water depths from 14 to 40 m. No substantial improvement in the relationship is observed by inclusion of benthic chlorophyll biomass and water temperature, apparently because variations in benthic chlorophyll concentrations are moderated by physical mixing processes and seasonal temperature ranges are relatively modest. Best fit for the data set is described by a simple power function [mg carbon fixed m À2 d À1 = 132(mol photon m À2 d À1 ) 1.45 ]. In situ, continuous PAR measurements indicate temporal variations and seasonal trends not revealed by previous ship-based measurements. Importantly, calculations of PAR flux and resulting BPP are significantly less than earlier estimates of benthic PAR derived via local insolation and water column light attenuation from ship-based light profiling. Furthermore, benthic PAR flux based on continuous above-water and near-surface inwater PAR measurements fails to correctly reflect seasonal trends for near-bottom PAR. Analysis of the time series data suggests that estimates of the vertical attenuation coefficient for PAR (k PAR ) values from ship-based measurements are biased to calmer conditions while variations in the vertical profile of light attenuation prevent accurate estimates of benthic PAR from near-surface or above-water measurements. Accurate estimates of the benthic light field and BPP over the annual cycle require in situ, continuous, high-resolution near-bottom measurements provided by ocean observatories.

In situ determination of porewater gases by underwater flow‐through membrane inlet mass spectrometry

“…However, in contrast to existing optode or electrode systems, the UMS is capable of simultaneously resolving a variety of dissolved gases, thus increasing the potential range of inference about biogeochemical properties. Elevated N 2 concentrations at depth at this site are expected considering the significant rates of denitrification measured in flow-through core incubations (Rao et al 2007(Rao et al , 2008. However, given the uncertainties generated by observations of gradients in argon profiles, it is difficult to draw conclusions about the importance of denitrification on the SAB shelf from the present observations.…”

“…Porosity of the sediment was given as 0.37 (Rao et al 2007(Rao et al , 2008. Tortuosity was calculated from porosity (Boudreau 1996).…”

“…Organic matter deposition and nutrient recycling are driven by advective fluxes of bottom water through the permeable sand bed (Jahnke et al 2005). Surface sands also support efficient denitrification (Rao et al 2007(Rao et al , 2008.…”

Quantifying photosynthetic rates of microphytobenthos using the triple isotope composition of dissolved oxygen

“…This is in the range of the rates of 6 to 450 mg C m -2 d -1 by MPB that have been observed in Naragansett Bay (Lake and Brush 2011), and the rates of 50 to 750 mg C m -2 d -1 by MPB that have been measured on the Georgia continental shelf (Jahnke et al 2000;Stanley and Howard Quantifying microphytobenthic photosynthesis Jahnke et al 2008). Additionally, the shape of the GOP curves in the light and water chambers tracked the incoming photosynthetically active radiation (PAR), providing support that this method quantifies photosynthesis (Fig.…”

“…Furthermore, there has been some debate as to whether radiocarbon-based productivity estimates are actually measuring gross or net productivity or something in between (e.g., Dring and Jewson 1982). Oxygen fluxes from incubations of cores in light and dark conditions in labo-ratories (Hopkinson et al 1999;Risgaard-Petersen 2003;Sundbäck et al 1991), in benthic flux chambers (Cebrian et al 2008;Jahnke et al 2000;Jahnke et al 2008), or from eddy correlation technique (Berg et al 2003;Glud et al 2010;Lorrai et al 2010;McGinnis et al 2008) are also used to estimate gross and net productivity. However these approaches require the assumption that light and dark respiration are similar, when in fact MPB light respiration appears to be greater than dark respiration by a factor of 1.5 to 2.5 (Epping and Jorgensen 1996;Fenchel and Glud 2000).…”