Abstract
Intense sampling of an estuary can reveal relative spatial changes that are significant irrespective of whether or not the estuary is eutrophic, micro- or meso-tidal, disturbed, or restored. This ‘waterscape’ perspective is analogous to a landscape perspective. We collected monthly water samples in the Barataria Basin watershed from 1994 to 2016 at 37 stations along a 129 km transect from 1 km offshore to a freshwater stream. The average Chlorophyll a (Chl) concentration from 267 trips was supported from both nitrogen-fixing cyanobacteria in a freshwater lake and partially from nutrients in seaward sources. Estuarine salinity was correlated with the discharge of the nearby Mississippi River. The main form of N was as organic nitrogen, not inorganic forms that recycle quickly, making changes in inorganic nitrogen concentration an unreliable indicator of net denitrification or uptake. The total nitrogen (TN) and total phosphorus (TP) concentrations declined with dilution towards the coast, but not because of denitrification. The phytoplankton standing biomass reflected the TN:TP ratio in the water column and there was a significant rise in the variability of Chl concentration at 2–6 psu, which was otherwise unremarkably constant. These waterscape patterns and cautionary interpretations may be common to other estuaries.
Highlights
Modern nutrient enrichment in estuaries and coastal waters is recognized as a causal agent of noxious phytoplankton blooms, fish kills, hypoxic zones, shellfish bed closures, and reduced coral reef and seagrass habitat (NRC, 2000; Cloern et al, 2001; Rabalais, 2002; Cloern & Jassby, 2008; Howarth et al, 2011)
We summarized measurements for individual stations and for all stations combined and tested five hypotheses (HO): (HO1) variations in inorganic and organic N forms from one station to another reveal hot-spots of rapid changes in nutrient cycling, (HO2) denitrification rates, if significant, will be evident in the disproportionate decline in total nitrogen (TN) compared to total phosphorus (TP); (HO3) variations in the Chlorophyll a (Chl) concentration and the frequency of algal blooms are coincidental with the molar TN:TP ratios the water column, (HO4) the system variance along the freshwater to sea continuum is constant, and (HO5) the offshore waters mix with inshore waters sufficiently to control estuarine salinity as measured by the gross changes from one year to the
Some of the Mississippi River discharge from Southwest Pass mixes within the estuary at Caminada Pass, Barataria Pass, Pass Coup Abel, and Four Bayous Pass to alter the salinity of the lower Barataria Bay and brings an additional nutrient loading to the estuary (Wiseman et al, 1990; Wissel et al, 2005)
Summary
Modern nutrient enrichment in estuaries and coastal waters is recognized as a causal agent of noxious phytoplankton blooms, fish kills, hypoxic zones, shellfish bed closures, and reduced coral reef and seagrass habitat (NRC, 2000; Cloern et al, 2001; Rabalais, 2002; Cloern & Jassby, 2008; Howarth et al, 2011). Numerous other examples are in Europe, Asia, and Australia (Zingone et al, 2010). Focused appraisals of these water quality changes are evolving to put the generic issue of biological eutrophication within a larger socio-politico framework (Cloern et al, 2001, 2016); the developing social consensus has resulted in the flourishing of some successful restoration measures (Duarte et al, 2009; Ruhl & Rybicki, 2010; Staehr et al, 2017). Water quality monitoring programs continue to track these changes and the consequences to help devise rehabilitation strategies and adaptations, and to evaluate new categories of change—climate, pharmaceuticals, dredging, and sediment diversions
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