Interactions of Salinity, Marsh Fragmentation and Submerged Aquatic Vegetation on Resident Nekton Assemblages of Coastal Marsh Ponds

Abstract

Increases in relative sea level are fragmenting the emergent vegetation of Louisiana's coastal marshes. Nekton abundance is likely impacted by salinity and whether emergent vegetation is replaced by submerged aquatic vegetation (SAV) or open water. To assess these effects, we sampled nekton densities along a salinity gradient (categorized as freshwater, intermediate, and brackish marsh) in fragmented and non-fragmented areas. Total nekton density increased strongly with SAV in brackish marsh but only weakly in freshwater marsh (F 2,238 =10.03, p< 0.0001). Freshwater and intermediate marshes had higher nekton densities when fragmented than when nonfragmented; this relationship was reversed in brackish marsh (F 2,238 =8.89, p=0.0002). Fragmentation, SAV, and salinity interacted to affect the densities of Gambusia affinis, Poecilia latipinna, Cyprinodon variegates, and Lucania parva. Our results suggest that the presence of both emergent vegetation and SAV was necessary for maintaining high nekton densities, with this combination being especially important in brackish marshes.

Nekton Density Patterns in Tidal Ponds and Adjacent Wetlands Related to Pond Size and Salinity

“…Zimmerman et al (1990) noted a similar shift by penaeid shrimps and blue crab in habitat use from marsh to open water where SAV occurred in upper Galveston Bay. Submerged aquatic vegetation can be an important habitat for nekton within the estuary (Weaver and Holloway 1974;Castellanos and Rozas 2001;Akin et al 2003;Kanouse et al 2006;Rozas and Minello 2006;Hitch 2009). Clearly, the distribution of SAV was important in structuring nekton density patterns within the intermediate zone at the time of our study, but in some years, this habitat type could also influence nekton density patterns down estuary.…”

“…Clearly, the distribution of SAV was important in structuring nekton density patterns within the intermediate zone at the time of our study, but in some years, this habitat type could also influence nekton density patterns down estuary. Although SAV coverage within marsh ponds in Louisiana is generally inversely related to salinity (Chabreck 1971) and the occurrence of SAV is temporally quite variable, it often occurs within brackish ponds (Chabreck 1971;Merino et al 2005;Kanouse et al 2006;Hitch 2009). At least one species (R. maritima L.) of SAV also occasionally occurs in ponds of the saline zone (Adair et al 1994;Merino et al 2009; LPR personal observation).…”

“…We could see density patterns within these areas much like we observed for the intermediate zone in our study, where animals shifted from emergent vegetation in marsh to SAV within ponds. Moreover, the habitat value of SAV in ponds may increase in brackish and saline zones with the loss of emergent wetlands (Hitch 2009).…”

“…We also used DTREG Predictive Modeling Software to run univariate classification and regression tree (CART) analysis to further explore potential relationships between selected fishery species and environmental characteristics (www.dtreg.com, P. H. Sherrod, Brentwood, Tennessee, 2003-2009. Each univariate CART analysis included a response variable of transformed (ln+1) density data for brown shrimp, white shrimp, or blue crab, three categorical explanatory variables (zone, pond size, habitat type), and seven continuous explanatory variables (water temperature, salinity, DO, water depth, turbidity, SAV coverage, stems).…”

“…Kanouse et al (2006) examined nekton populations within ponds of a brackish marsh in Louisiana by sampling submerged aquatic vegetation (SAV) and shallow nonvegetated bottom (SNB) within eight marsh ponds, but they did not sample the emergent marsh habitat along the pond shoreline. Hitch (2009) examined the distribution of nekton within ponds along environmental gradients of salinity, marsh fragmentation, and SAV coverage but did not examine the effect of pond size on nekton density patterns. We are unaware of any published studies comparing the habitat value of tidal ponds and adjacent wetlands that were designed to examine gradients of both pond salinity and pond size.…”