High secondary production in a Coastal Plain river is dominated by snag invertebrates and fuelled mainly by amorphous detritus

Abstract

Summary The 2‐years production dynamics of major taxa (family/order) and functional feeding groups was synthesised from detailed production analyses previously published for all major invertebrate species/genera (retained by a 100 μm sieve) found on snags (submerged wood) in a sixth‐order south‐eastern U.S. Coastal Plain river. The trophic basis of this production and annual consumption of six food types were quantified. Invertebrate production (148 and 203 g dry mass m−2 year−1 of snag surfaces for the 2 years) was among the highest ever measured for a specific habitat in non‐polluted or unregulated rivers and was still relatively high when converted to overall area of the river bed (48 and 70 g m−2 year−1). Snag production constituted 73% of total main‐channel production (production from snag and river bed combined). Chironomidae (40–44% over the 2 years), Ephemeroptera (14–21%) and Trichoptera (25–31%) together represented the majority (88–93%) of the putative production of primary consumers on snags. Functional group analysis showed that fine filterers (26–36% over the 2 years), gatherers (36–44%) and macro‐filterers (21–25%) represented the majority of total invertebrate production. Production of predaceous insects (mostly Plecoptera, Odonata and Megaloptera) represented only 5–7% of the total. Production of the three major taxa and three major functional groups was remarkably consistent over 2 years of monthly sampling, despite strong seasonal variation in discharge/light/temperature and production of individual genera and families. Seventy‐nine per cent of production by primary consumers was supported by microbially enriched amorphous detritus, the major component of seston originating primarily from floodplain forests. Invertebrates consumed ˜357 g AFDM m−2 (river bed) year−1 of seston, 325 g m−2 year−1 of amorphous detritus and 20 g m−2 year−1 of diatoms. Dividing seston consumption by seston transport indicated that ˜67% of seston was consumed over 100 km of river length. Consumption was higher than autochthonous net primary production, supporting other evidence that terrestrially derived dissolved organic matter, incorporated via the microbial loop, is the main trophic basis of this riverine food web. Macro‐filtering hydropsychid caddisflies were omnivorous with > half their production from carnivory. Snag predators, including hydropsychids, consumed ˜60% (29 g m−2 year−1 of river bed) of invertebrate production (assumed to be from snags), leaving the remainder for emergence, downstream transport and consumption by fish.