Physical and biogeochemical controls on light attenuation in a eutrophic, back-barrier estuary

Abstract

Abstract. Light attenuation is a critical parameter governing the ecological function of shallow estuaries. In these systems primary production is often dominated by benthic macroalgae and seagrass; thus light penetration to the bed is of primary importance. We quantified light attenuation in three seagrass meadows in Barnegat Bay, New Jersey, a shallow eutrophic back-barrier estuary; two of the sites were located within designated Ecologically Sensitive Areas (ESAs). We sequentially deployed instrumentation measuring photosynthetically active radiation, chlorophyll a (chl a) fluorescence, dissolved organic matter fluorescence (fDOM; a proxy for colored dissolved organic matter (CDOM) absorbance), turbidity, pressure, and water velocity at 10 min intervals over 3-week periods at each site. At the southernmost site, where sediment availability was highest, light attenuation was highest and dominated by turbidity and to a lesser extent chl a and CDOM. At the central site, chl a dominated followed by turbidity and CDOM, and at the northernmost site turbidity and CDOM contributed equally to light attenuation. At a given site, the temporal variability of light attenuation exceeded the difference in median light attenuation between the three sites. Vessel wakes, anecdotally implicated in increasing sediment resuspension, did not contribute to local resuspension within the seagrass beds, though frequent vessel wakes were observed in the channels. With regards to light attenuation and water clarity, physical and biogeochemical variables appear to outweigh any regulation of boat traffic within the ESAs.

Short-Term Dissolved Organic Carbon Dynamics Reflect Tidal, Water Management, and Precipitation Patterns in a Subtropical Estuary

“…As sea level rise drives higher saltwater intrusion and increased inundation in this region, higher C loss from the mangrove forest (See also Section Mangrove Contributions to the DOC Pool) is expected to account for a larger portion of the Shark River DOC pool and increase outwelling of mangrovederived DOC (Dittmar and Lara, 2001). Such processes are expected to increase light attenuation in the water column (Ganju et al, 2014) of adjacent light-sensitive seagrass and coral reef communities (Shank et al, 2010;McPherson et al, 2011) through long-range DOC transport (Maie et al, 2012;Yamashita et al, 2013. However, DOC export from this region in general is predicted to decrease in the future due to shifting hydrology due to climate change (Regier et al, 2016).…”

“…DOC also influences biogeochemical cycling within estuaries, including regulating the transport of nutrients and metals (Boyer et al, 1997;Qualls and Richardson, 2003;Yamashita and Jaffé, 2008) and impacts microbial communities (Tranvik, 1998;Fellman et al, 2010). Components of the DOC pool which absorb UV light, known as chromophoric dissolved organic matter (CDOM), control benthic productivity through light attenuation in the water column (Osburn et al, 2009;Ganju et al, 2014).…”

“…For instance, high-frequency FDOM measurements have facilitated the development of ecosystemspecific proxies for mercury , nutrients (Wilson et al, 2013;Etheridge et al, 2014), and DOC lability (Wilson et al, 2013). Likewise, high-resolution characterization of DOC proxies have provided new insight into processes controlling diel carbon cycles (Watras et al, 2015), rapid changes in hydrologic export of DOC Wilson et al, 2013) and the role of DOC in light attenuation within optically complex waters (Ganju et al, 2014). As such, the application of instruments capable of high-frequency DOC data collection holds great potential for advancing our understanding of temporally dynamic aquatic ecosystems.…”

Colored dissolved organic matter in shallow estuaries: relationships between carbon sources and light attenuation

“…For the purpose of this study, sites north of Barnegat Inlet are referred to as "North Barnegat Bay", while sites parallel to and south of Barnegat Inlet are referred to as "South Barnegat Bay". Tides are semidiurnal and range from < 0.1 to 1.5 m, and current velocities range from < 0.5 to 1.5 m s −1 (Kennish et al, 2013;Ganju et al, 2014); there is also a pronounced south-to-north gradient in tidal range and flushing (Defne and Ganju, 2014). While the land surrounding the northern portion of the bay is developed with mixed urban-residential land use, the area south of Barnegat Inlet is less developed and retains much of the original marsh (Wieben and Baker, 2009).…”

“…where A(λ) is the absorbance at 340 nm, l is the cell length in meters (0.05 m for this study), and a(λ) is the absorption coefficient (Green and Blough, 1994). Absorbance values at 340 nm were the highest across the range scanned, so 340 nm was chosen as the absorbance wavelength for calculating the absorption coefficient.…”