Primary production, carbon flux and the distribution of the amphipod Diporeia in Lake Superior

The amphipod Diporeia is an important feature of the Great Lakes food web, figuring prominently in the diet of several commercially important fish species (e.g., lake whitefish and lake trout). Once widely distributed across the Great Lakes, Diporeia populations are now in severe decline or collapse (DERMOTI & KEREC 1997, NALEPA et al. 1998, 2001, LOZANO et al. 2001) in all but Lake Superior (AUER & KAHN 2004, SCHAROLD et al. 2004). Competition for food resources with invasive mussels (Dreissena) has been cited as a likely reason for the decline in Diporeia; however, no cause-effect relationship is as yet widely accepted. Healthy populations ofthe amphipod in Lake Superior offer an opportunity to study the natural history and energy dynamics of this organism and to provide insights regarding factors mediating extirpation ofthe phylogroup in the other Great Lakes. Research on Great Lakes benthos (e.g., NALEPA 1989, EvANS et al. 1990), including Diporeia, indicates that populations reach higher densities in slope habitats ( depth of30-125 m) than in shallower (0-30 m, shelt) or deeper (>125 m, profundal) regions. Density maxima have been observed in Lake Superior at depths of 40-100 m (CooK 1975, KRAFI 1979, AUER & KAHN 2004) an d the presence o f shelf-slope-profundal differences in Diporeia biomass confirmed (N. Auer, unpubl.; Fig. l inset). Subsequent surveys (N. Auer, unpubl.) have demonstrated that such distributions are widespread in Lake Superior. Some investigators have suggested thatthe density peak may coincide with a bando f sedimented organic ma tter originating from onshore sources (MozLEY & ALLEY 1973). These findings have led our research group to postulate the existence o f a "ring o f fire" in Lake Superior, anarrow band located in nearshore waters where enhanced primary production supports a robust macroinvertebrate population and the attendant secondary production required by species higher in the food web. The location ofthat ring offire is thought to be essentially coincident with the lake 's slope region, but varying in width among locations due to interactions between wind driven turbulence and the depositional environment. The central hypothesis ofthis work is that peaks in amphipod numbers in the slope region are associated with enhanced availability of energy resources. Rates of particulate organic carbon deposition, assessed using sediment traps, are documented in this study to be 2-5 times greater in Lake Superior's slope region than in the adjacent profundal region. Direct deposition and post-depositional focusing can be expected to deliver these energy resources to amphipod populations occupying the slope. Profundal environments would receive lesser amounts due to consumption by benthos during focusing and due to processing of particulate matter during sedimentation (longer residence time; see SIERSZEN et al. 2006). Variation in primary production driven by differences in nutrient supply, temperature, and light conditions (mixing depth) may also contribute to the availability of energy resources in slope and profundal habitats. In this study we quantified areal net organic carbon production for nearshore sites potentially delivering food resources to the slope region and for offshore sites serving a similar function for the profundal region. The results of these calculations were used to examine the ro le of nearshore-offshore differences in primary production in goveming heterogeneity in amphipod distribution.