Great Lakes coastal wetland fragmentation: Edge effects on zooplankton, macroinvertebrate, and larval fish communities.

摘要

Wetlands are some of the most diverse and ecologically important habitats in the world. Although wetlands have been plagued by anthropogenic destruction and disturbance, there is a general lack of knowledge regarding how fragmentation affects these systems. During the summer of 2004 and the spring and summer of 2005, I investigated edge effects associated with anthropogenic fragmentation in 25 fringing marshes of Lakes Michigan and Huron. Environmental data, microcrustacean zooplankton, aquatic macroinvertebrates, and larval fish were collected along transects extending into each marsh from reference (natural) and anthropogenic (artificial) edges. Bulrush stems of Great Lakes fringing marshes attenuated the influx and mixing of wind-driven pelagic water. Thus aeration, turbidity, and organic extraction declined from edge to marsh interior. The accumulation of organic material toward the inner marsh from any type of edge was associated with gradients of dissolved oxygen concentration, pH, and alkalinity. Invertebrate and larval fish communities responded to hydrologic mixing gradients associated with reference and anthropogenic edges. Overall, the invertebrate community exhibited positive spatial trends in total abundance, biomass, richness, and Shannon diversity from either edge type toward the inner marsh. The larval fish community also showed increasing gradients of total abundance, richness, and Shannon diversity inward from both edge types. Larval largemouth bass (Micropterus salmonoides), larval yellow perch (Perca flavescens), and larval banded killifish (Fundulus diaphanous) were associated with protected, inner marsh habitat. Overall, more common carp (Cyprinus carpio) larvae were caught throughout the anthropogenic gradient zone than in the natural gradient zone. Thus, anthropogenic edge creation may promote higher densities of the invasive, habitat-destroying common carp in Great Lakes wetlands. Invertebrate and larval fish communities inhabiting anthropogenic edges and anthropogenic gradient zones that experienced relatively more wind-induced hydrologic mixing intensity were less like inner marsh communities. Therefore, marshes that are exposed to a higher level of wind-induced hydrologic energy are likely more susceptible to biological community change when an anthropogenic edge is created. As new edges are created within fringing marshes, anthropogenic gradient zones gain area at the expense of inner marsh habitats. I observed that when a marsh fragment's size falls below a minimum threshold, the relatively dense inner core area is replaced by anthropogenic gradient zones comprised of thinly-spaced stems. The marsh remnant can no longer maintain a significant chemical and physical contrast to the overwhelming influx of pelagic water from the edges. Organic debris and submerged vegetation are consequently lost to pelagic extraction and long-shore currents. Without the unique, complex habitat of a protective inner core, a marsh remnant may cease to function as a viable refuge against hydrologic energy, large predators, and pelagic invaders, eventually losing all or most of its inner marsh invertebrate and larval fish taxa. The lower total abundance, taxa richness, and diversity of larval fish in small marshes suggests that spawning and nursery habitat of relatively vulnerable marsh remnants may be inferior to the protective inner habitat associated with large marshes.