Effects of plant community composition and exposure to wave action on invertebrate habitat use of Lake Huron coastal wetlands

Invertebrate communities from different coastal marsh-plant communities were compared along wave-exposure gradients using data from 1994, 1998 and 1999. Data were subjected to correspondence analyses to search for patterns in invertebrate communities in relation to plant-community structure and wave exposure. In 1994, quantitative plant- and sediment-invertebrate samples were taken from nine habitats: four from inland, subsurface-connected marshes and five from littoral, emergent marshes. In 1998, sweep-net samples were taken from 13 plant communities: six on the exposed and seven on the protected side of an island. In 1999, 2–3 plant communities/sites were sampled with sweep nets from four sites around the Bay so that intersite differences between inner, less-exposed and outer, more-exposed habitats could be examined. In all three studies, correspondence analyses separated inland, protected or inner sites from littoral, exposed or outer sites, suggesting differences in invertebrate-community structure. For example, Hydracarina and Asellidae occurred in large numbers in inland sites, but were less common or absent from exposed, littoral sites. Littoral marshes also separated along an exposure gradient with Tanytarsini and Orthocladiinae collectors of organic particles occurring in very high numbers in outer, exposed areas where organic particles from the pelagic zone entered the marsh. Certain plant-community types clustered together (e.g. wet meadow and Scirpus) while others, such as Typha, stands clustered according to exposure to waves suggesting the importance of both plant-community structure and wave exposure in determining invertebrate-community structure. We present a conceptual model that suggests that invertebrates in Great Lakes' marshes are distributed along gradients of decreased mixing of pelagic water and increases in sediment organic matter from outer to inner marsh and between littoral and adjacent inland marshes. Some invertebrates do best on one end of these gradients, while the majority are generalists found across habitat types.