Intertidal benthic resources of the Copper River Delta, Alaska, USA

Abstract

The Copper River Delta, Alaska is the largest contiguous coastal wetland system along the West Coast of North America. Vast expanses of tidal mud flats formed by sediments carried by the suspended load of the Copper River serve as a connection between the Gulf of Alaska and the extensive network of wetlands, rivers and sloughs of the delta system. In addition to providing habitat for resident fish, shrimp and crabs, these tidal flats serve as critical feeding grounds for up to 5 million migratory shorebirds as well as an entry and exit corridor for three species of commercially fished salmonids. Here we report the first description of the benthic community of these intertidal flats. Between April and September 2000, we conducted three samplings on the Copper River Delta in which we quantified benthic macro-invertebrates inhabiting silt-clay sediments, the dominant substrate in the system, over a range of tidal inundation. Specifically, sampling was performed in two areas on the delta: near the outflows of the Eyak River and Pete Dahl Slough. Pore-water salinity of surficial sediment ranged from 4 psu during peak summer flow of the Copper River to 14 psu in April prior to increased riverine input. Sediment temperatures corresponded to ambient air temperatures with lowest temperatures during the April–September observation period recorded in April (4°C) and warmest in August (16°C). The benthic community of the delta's tidal flats was characterised by low species diversity and was dominated by the tellinid bivalve Macoma balthica, which reached densities greater than 4000 m −2. Age–length relationship of M. balthica indicated slow growth and longevity of up to 8 years. Polychaete densities, primarily the phyllodocid Eteone longa, were low throughout the study period, reaching a maximum of only 700 m −2 in August. Amphipod densities, primarily the corophid amphipod Corophium salmonis, were high (up to 7000 m −2) only during the August sampling. Spatial patterns of benthic invertebrate abundance were best explained by differences in tidal inundation with longer inundation corresponding to greater invertebrate densities. Temporal changes in abundance of polychaetes, amphipods, and M. balthica recruits corresponded to increases in sediment temperatures. Natural or human-induced changes to M. balthica populations could impact the food web of the delta, which could cascade to larger geographic impacts because of the importance of the delta to migratory species.