Effects of hydrologic connectivity on aquatic macroinvertebrate assemblages in different marsh types

Abstract

Hydrologic connectivity can be an important driver of aquatic macroinvertebrate assemblages. Its effects on aquatic macroinvertebrate assemblages in coastal marshes, however, are relatively poorly studied. We evaluated the effects of lateral hydrologic connectivity (permanently connected ponds: PCPs; temporary connected ponds: TCPs), and other environmental variables on aquatic macroinvertebrate assemblages and functional feeding groups (FFGs) in freshwater, brackish, and saline marshes in Louisiana, USA. We hypothesized that (1) aquatic macroinvertebrate assemblages in PCPs would have higher assemblage metric values (density, biomass, Shannon-Wiener diversity) than TCPs and (2) the density and proportional abundance of certain FFGs (i.e. scrapers, shredders, and collectors) would be greater in freshwater marsh than brackish and saline marshes. The data in our study only partially supported our first hypothesis: while freshwater marsh PCPs had higher density and biomass than TCPs, assemblage metric values in saline TCPs were greater than saline PCPs. In freshwater TCPs, long duration of isolation limited access of macroinvertebrates from adjacent water bodies, which may have reduced assemblage metric values. However, the relatively short duration of isolation in saline TCPs provided more stable or similar habitat conditions, facilitating higher assemblage metric values. As predicted by our second hypothesis, freshwater PCPs and TCPs supported a greater density of scrapers, shredders, and collectors than brackish and saline ponds. Aquatic macroinvertebrate assemblages seem to be structured by individual taxa responses to salinity as well as pond habitat attributes. KEY WORDS: Coastal marsh · Aquatic macroinvertebrate assemblage · Hydrologic connectivity · Functional feeding groupsResale or republication not permitted without written consent of the publisher

Seasonal comparison of aquatic macroinvertebrate assemblages in a flooded coastal freshwater marsh

“…The highest SPWD was in fall. The full results of water chemistry (i.e., salinity, DO, temperature), SAV cover, and SPWD in freshwater marsh ponds (i.e., PCPs and TCPs) are presented in Table 1 and [17].…”

“…Our results, however, were not consistent with a hydroperiod effect on assemblage structure as we did not detect a difference in dominant species between ponds and freshwater emergent marsh. Based on [17], most aquatic macroinvertebrates in these coastal marshes appear to be poorly adapted to dry conditions. In freshwater emergent marsh, midges (Chironomidae) were relatively abundant species (i.e., >10 %) during all flooded seasons.…”

“…According to previous studies [16,18,19], odonates require a relatively long hydroperiod for the full development of nymphs; thus, they may avoid the marshes due to the risk of dry conditions. However, coleopterans have relatively high desiccation tolerance; therefore, they may avoid the ponds because they support an abundance of relatively large and strong predators (e.g., fish, dragonfly; [17]). Despite differences in physical and chemical conditions between marshes and ponds, our results indicated that the density of odonates and coleopterans between marshes and ponds did not statistically differ.…”

“…Coleopterans are known to possess physiological and behavioral mechanisms to survive desiccation during dry periods (e.g., Dytiscidae; [16]) and these traits may allow them to avoid the deep water habitat that commonly support relatively large and strong predators (e.g., fish, odonates; [17]). Thus, coleopterans in freshwater emergent marsh may be more abundant than in ponds.…”

“…In addition, macrophyte (e.g., SAV: submerged aquatic vegetation) cover appears to affect macroinvertebrate distribution by providing refuge from predators [20], and increasing the availability of food resources [21]. [17] characterized aquatic macroinvertebrate assemblages in freshwater marsh ponds, however, a paucity of information [22] exists on the aquatic macroinvertebrate assemblages in freshwater marshes and their similarity to assemblages in freshwater marsh ponds.…”