2010 S.C. Water Resources Conference - Science and Policy Challenges for a Sustainable Futur

Bergquist, Derk

Brock, Larissa M

Felber, Jordan

Greenfield, Dianne I

Keppler, Charles

Clemson University: TigerPrints

The Influence of Nutrient Loading on Seasonal Phytoplankton Community Variability in a TemperateEstuary: Case Study of the Ashepoo Combahee Edisto(ACE) Basin, South Carolina, USALarissa M. Brock1, Derk Bergquist1, Jordan Felber1, Charles Keppler1 and Dianne I. Greenfield1,2,1 Marine Resources Research Institute, South Carolina Department of Natural Resources, 217 FortJohnson Road, Charleston, SC 29412; brockl@dnr.sc.gov2Belle W. Baruch Institute for Marine and Coastal Sciences, 607 EWS Building, University of SouthCarolina, Columbia, SC 29208The Ashepoo Combahee Edisto (ACE) Basin (South Carolina, USA), part of the National EstuarineResearch Reserve (NERR) system, drains water from ~135,000 acres (~545 km2) of South Carolina. Landuse within the ACE Basin includes natural forest, silviculture, agriculture, residential development,managed wetlands, and other terrestrial categories. In the past, the ACE Basin has been assumed to be‘pristine’ relative to other temperate estuaries both within South Carolina and the US. However, recent(1999 present) monitoring and research by the SC Estuarine and Coastal Assessment Program (SCECAP),a state wide monitoring program that assesses the environmental health of SC coastal embayments,revealed areas of elevated nutrients (total phosphorus (TP) and nitrogen (TN)), fecal coliform, and lowerdissolved oxygen within the ACE Basin compared to other South Carolina estuaries (Van Dolah et al.2006). Additionally, SCECAP data showed that chlorophyll a levels often followed trends of TN and TP,suggesting that phytoplankton may be responding to total (inorganic and/or organic) nutrientenrichment. Furthermore, chlorophyll a levels during 1999 2000 were considered moderately eutrophicusing the Bricker et al. (1999) classification (Nobel et al. 2003). In order to follow up on SCECAP findings,elucidate potential pollutant sources, and determine seasonal phytoplankton responses to nutrientlevels, an intense, multi phase monitoring and experimental effort was initiated in the ACE Basin duringthe summer of 2008. Here we discuss findings from the first two phases: Phase I) assessment of waterquality and land use analysis; Phase II) a focused watershed scale, targeted system monitoring studyincluding nutrient addition bioassays within three target systems. The third phase (beginningSeptember 2010) includes additional bioassays, fecal coliform sampling at managed wetland outfallsinto receiving tidal creeks, and outreach.During Phase I (summer of 2008), 60 randomly selected tidal creek and open water sites (30each) and 7 additional stations determined ‘poor’ habitat according to SCECAP criteria, (top 10percentile in concentrations statewide) were sampled for water quality, (temperature, salinity, dissolvedoxygen, pH and secchi measurements), TN, TP, dissolved organic carbon (DOC), chlorophyll a, totalcoliform, Escherichia coli, Enterococcus spp., and total/volatile suspended solids. Results indicated thatsystems with elevated bacterial levels coincided with systems where nutrient levels also tended to behigh, relative to other ACE Basin regions. Creek systems not previously identified by SCECAP efforts alsoexhibited relatively elevated (when compared to other SC systems) bacteria and total nutrient levels,suggesting that they share a common source or that they are influenced by a common suite ofenvironmental conditions. Using Geographic Information Systems (GIS; ESRI 9.3) to define land usecharacteristics surrounding the stations, it was determined that land cover was dominated by salt marsh(~55% of total area), followed by open water and upland (primarily evergreen forest). Freshwater andbrackish marsh (category including managed wetlands) generally accounted for a small percentage oftotal land cover. Based on land use analyses for the random stations sampled, nutrient and bacteriallevels were positively and often significantly correlated (p<0.05, stepwise regression) with the presenceof freshwater wetlands (often occurring as managed wetlands) and to a lesser extent forested upland.These same measures were negatively correlated with the area of estuarine open water and none of theparameters were significantly correlated with salt marsh area. These data suggest that salt marsh coverdoes not explain the variability in water quality parameters within the ACE Basin and upland andfreshwater/brackish marsh systems influence (possibly through inputs from water run off) the dilutioncapacity thereby contributing to the overall water quality variability.Based on Phase I findings, ten target systems were chosen for seasonal sampling during Phase II.Seven systems exhibited the highest levels of nutrient and bacteria load in Phase I and three controlsystems had comparatively low levels of bacteria and nutrients. The systems represented various landuse categories (wetland, forested, salt marsh). Four stations (headwaters, two roughly equidistantmiddle stations and the mouth) were sampled within each system at ebb tide. The same environmentaland water quality parameters mentioned above were measured, in addition to dissolved inorganicnitrogen (nitrate/nitrite and ammonium) and phosphorus (orthophosphate). All of the creek systemswith substantial human land use activity (urban/suburban development, managed wetlands, agricultureetc.) showed elevated levels of total coliforms, Escherichia coli, Enterococcus spp. microbial indicatorsrelative to creeks draining watersheds with little to no human impact. Some systems also exhibitednearly hypoxic dissolved oxygen levels throughout Phase II. Generally, inorganic nutrient levels werelow relative to total nutrients suggesting that organic nutrients (particularly dissolved) could largely becontributing to the productivity of these systems. Nitrogen to phosphorus ratios were typically low (N:P<16), particularly in the inorganic fractions, suggesting that the systems are either inorganic nitrogenlimited or are orthophosphate replete.Additionally, during Phase II we experimentally assessed the short term (50 hr) growthresponses of phytoplankton to inorganic nutrient enrichment using bioassays conducted seasonally June2009 – March 2010 within three of the ten targeted systems, each representing different land usepatterns. The control site, the New Chehaw River, primarily drains salt marsh and unmanagedforested/marsh inlands. The Old Chehaw River drains land characterized by managed wetlands,unmanaged forest, and agriculture. Both the New and Old Chehaw Rivers drain into the CombaheeRiver. The third site, located in Sampson Creek, is surrounded by several large managed wetlands anddrains into the South Edisto River. At each site, 50 hour nutrient bioassays were conducted in situ usingsurface (0.3 m) samples (1L in triplicate) containing natural phytoplankton assemblages collected at midebb. Five treatments were amended with ammonium, nitrate, phosphate, ammonium + phosphate,nitrate + phosphate and an unamended control. Net growth for phytoplankton was calculated fromtotal and <20 uM chlorophyll a size fractions. Briefly, low N:P (< 16) in the New Cheehaw River,combined with phytoplankton growth rates being typically enhanced relative to the control wheninorganic nitrogen and phosphorus were added, suggests that this system is generally N limited (Penriched). Phytoplankton did not respond to nutrient additions in Sampson Creek and showed anintermediate response in the Old Cheehaw relative to controls. The comparatively low phytoplanktongrowth rates at Sampson Creek were likely related to elevated total nutrients (particularly TN),indicating saturated conditions. Low levels of dissolved inorganic nutrients relative to totals furthersuggest that organic constituents may be the key drivers of ACE Basin productivity. Microbial utilizationof organic matter is directly linked to hypoxia in many estuarine systems, and may be vital for driving thenear hypoxic to hypoxic conditions observed in the ACE basin, but further investigation is needed.The first two phases of the ACE basin study have confirmed that areas within the ACE basin donot exhibit ‘pristine’ water quality characteristics as once presumed. Enhanced phytoplankton growthfor N additions, combined with N:P typically <16, suggests that the ACE Basin and associatedphytoplankton communities are primarily N limited. Moreover, P is generally not a co limiting nutrient.These data suggest that phytoplankton community dynamics of the ACE Basin may be influenced byvarying concentrations of N and P. Furthermore, overall nutrient levels may be associated with regionalland use patterns. Further information is needed to determine the source of the elevated bacterialevels and if organic nutrients are overall contributer to the nutrient forms. The focus of Phase I and IIbacteria sampling was total coliform, Escherichia coli, Enterococcus spp. and although high levels(relative to other ACE Basin systems) of these forms were found in the systems studied, fecal coliformlevels will be researched in Phase III to conform with state shellfish bed water quality standards set bythe Department of Health and Environmental Control. To determine if organic fractions are the majorcontributor to the nitrogen load, nutrient addition bioassays will be expanded by adding organicfractions (in the form of urea) to the in situ experiments. To facilitate efficient transfer of scientificinformation to land managers, a workshop for wetland managers will also be conducted in Phase III.Citations:Bricker, S. B., C. G. Clement, D. E. Pirhalla, S. P. Orlando, and D. R. G. Farrow. 1999. National estuarineeutrophication assessment: effects of nutrient enrichment in the nation’s estuaries. National Oceanicand Atmospheric Administration, National Ocean Service, Special Projects Office, and the NationalCenters for Coastal Ocean Science, Siler Spring, Md.Nobel, P.A., R. G. Tymowski, M. Fletcher, J. T. Morris, and A. J. Lewitus. 2003. Contrasting patters ofphytoplankton community pigment composition in two salt marsh estuaries in southeastern UnitedStates. Applied and Environmental Microbiology. p. 4129 4143.Van Dolah, R.F., D.C. Bergquist, G.H.M. Riekerk, M.V. Levisen, S.E. Crowe, S.B. Wilde, D.E. Chestnut, W.McDermott, M.H. Fulton, E. Wirth, J. Harvey. 2006. The Condition of South Carolina’s Estuarine andCoastal Habitats During 2003 2004: Technical Report. Charleston, SC: South Carolina Marine ResourcesDivision. Technical Report No. 101. 70 p.

The Influence of Nutrient Loading on Seasonal Phytoplankton Community Variability in a Temperate Estuary: Case Study of the Ashepoo-Combahee-Edisto

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