Spatial and Temporal Patterns in Insect Emergence from a Southern Maine Salt Marsh

Article excerpt

ABSTRACT.-

Adult insects were collected using emergence traps from salt water pools, brackish water pools and vegetated marsh in the Little River Estuary, Wells, Maine, USA. Samples were continuously taken once every week from June through October of 2001 to examine the community composition, as well as the spatial and temporal patterns of insect emergence from a salt marsh system. Chironomus sp. and Tanytarsus spp. (Diptera: Chironomidae) chironomids were the most abundant insects emerging, the former representing 87% of all insects collected from brackish pools and the latter 55-80% of all insects collected from saltwater pools and the vegetated marsh, respectively. Insect emergence was higher from brackish pools (1450 ± 172 individuals.m^sup -2^.y^sup -1^) and vegetated marsh sites (1350 ± 851 individuals.m^sup -2^.y^sup -1^), but not significantly different from salt water pools (289 ± 126 individuals.m^sup -2^.y^sup -1^). Emergence of some species was restricted to certain marsh areas resulting in non-significant trends in patterns of community composition, diversity and evenness. More species (n = 15) were collected from the vegetated marsh compared to brackish (n = 13) and salt water pools (n = 11), while diversity and evenness were greater in salt water pools (H'= 1.56 ± 0.4, 0.5 ± 0.2) compared to the vegetated marsh (H' = 1.18 ± 0.8, 0.3 ± 0.2) and brackish pools (H' = 0.72 ± 0.01, 0.2 ± 0.02). Temporal patterns of the two most abundant chironomids revealed increased numbers emerging in June (Tanytarsus spp. and Chironomus sp.) and again in September (Chironomus sp.). This suggests the importance of salt marsh insects as a food source to fish and birds, particularly during breeding and nesting seasons in the spring (June) or migratory season in the fall (September).

INTRODUCTION

Benthic macroinvertebrates play important roles in estuarine ecology. For example, macroinvertebrates aid in the breakdown of upland- and salt marsh-derived organic matter (Graça et al., 2000; Zimmer et al, 2002), can affect plant community composition (Silliman and Zieman, 2001; Silliman and Bertness, 2002) and are an important food source for different life stages of resident and transient organisms (i.e., birds, fish) (Erwin, 1996; Deegan et al., 2000; Smith et al., 2000). The community structure of macroinvertebrates has also been well documented from several estuarine areas (e.g., mud flats, tidal creeks, channels and, to some extent, salt marsh sediments) (Larsen et al., 1983; Ruth et al, 1994; Sardá et al., 1998; Fell et al., 2003). One group that is often ignored, particularly in their level of taxonomic identification (Giberson et al, 2001), is aquatic insects. Despite the common presence of insects in salt marsh systems, few studies have focused on their spatial and temporal distributions (but see Menzie, 1981; Giberson et al., 2001; Milakovic et al., 2001), their diversity or their overall role in salt marsh ecology.

In freshwater wetlands, aquatic insects are a major component of the macrofaunal assemblage (Batzer and Wissinger, 1996; Wissinger, 1999; MacKenzie et al, 2004) where they function similarly to estuarine macroinvertebrates by aiding in organic matter breakdown (Smock and Harlowe, 1983; Batzer, 1998), serving as important food sources for other organisms (Hanson and Riggs, 1995; Beletsky and Orians, 1996), etc. High densities and turnover rates of larvae from Chironomidae (Diptera) in estuaries suggests insects may also be an important food source in salt marshes (Menzie, 1981; Milakovic et al, 2001). This is evident by the presence of aquatic insects in stomach contents of resident fish (Walsh and Fitzgerald, 1984; Smith et al, 2000; Warren et al, 2002) and observations of various birds feeding on insect larvae on the marsh surface (Erwin, 1996). However, due to the limited amount of research in these habitats, our understanding of the ecological value of aquatic insects in estuarine systems is limited. …