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

Abstract

Continuous, 7 year records of above‐water, near‐surface water column and near‐bottom 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−2 d−1 = 132(mol photon m−2 d−1)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 in‐water 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 (kPAR) 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.