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.
Highlights
Back-barrier estuaries, especially along the Atlantic coast of the United States, are typically shallow systems colonized by benthic primary producers (Ziegler and Benner, 1998; Meyercordt et al, 1999; McGlathery et al, 2001)
The platform consisted of an RBR D|Wave recorder; a pair of WetLabs ECO-PARSB self-wiping photosynthetically active radiation (PAR, 400– 700 nm) sensors; a YSI EXO multisonde measuring temperature, salinity, turbidity, chlorophyll a fluorescence, fluorescing dissolved organic matter, pH, and depth; and a Nortek Aquadopp-HR 1 MHz current profiler
The chl a demonstrated a diurnal signal (Figs. 2, 4), with troughs during peak daylight; this is a characteristic signature of non-photochemical quenching (NPQ) whereby chlorophyll fluorescence is reduced at high levels of irradiance (Fig. 5; Maxwell and Johnson, 2000; Lawrenz and Richardson, 2011)
Summary
Back-barrier estuaries, especially along the Atlantic coast of the United States, are typically shallow systems colonized by benthic primary producers (Ziegler and Benner, 1998; Meyercordt et al, 1999; McGlathery et al, 2001). Prior to urbanization and agricultural influence in the watersheds, many systems were characterized by high densities of seagrass and relatively low light attenuation. Subsequent nutrient loading resulting from industrial and agricultural development has led to a gradual decline in seagrass density as eutrophication created water-column algal blooms and overgrowth of benthic algae (Burkholder et al, 2007). Both of these algal succession processes reduce light penetration to seagrass colonies. There are generally four major factors that reduce light penetration in the water column: the water itself (a function of depth), non-algal particulate material (i.e., suspended sediment), phytoplankton biomass, and colored dissolved organic matter (CDOM) (Kirk, 1994).
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