Substantial pulses of aquatic insects emerge from tidal freshwaters along the James River Estuary, Virginia, USA

Tidal freshwaters in upper estuarine reaches provide important ecosystem services but are threatened by relative sea-level rise and pollution from increased development. Tidal freshwaters are highly productive and support estuarine and riparian food webs alike. Aquatic insects are common prey subsidies crossing into riparian habitats; however, the magnitude, timing, and composition of insect emergence in tidal systems has received little attention. Our objective was to better understand the magnitude and variability of aquatic insect emergence in tidal freshwaters. To do so, we quantified insect emergence from tidal creeks and estuarine shorelines of the James River Estuary, Virginia, USA, and characterized spatial and temporal patterns in the amount of emergent biomass. We continuously monitored insect emergence from 7 April to 8 November 2019 with floating emergence traps to estimate daily emergence, then used generalized additive mixed models to analyze spatial and temporal variation in daily emergence rates. We estimated aquatic insect biomass to emerge at a mean rate (+/- 1 SE) of 15.6 +/- 2.0 g dry mass m(-2) y(-1), which is among the highest of previously published estimates from nontidal systems (mean +/- 1 SE 5 12.9 +/- 6.2 g dry mass m(-2) y(-1)). Spatial variability in emergence was highly taxon specific. Diptera and Trichoptera had more biomass emerging from the subtidal than intertidal zone, Odonata biomass emerged more from tidal creeks than along the estuarine shoreline, and the amount of Trichoptera biomass increased-whereas Ephemeroptera decreased-with distance from the estuarine shoreline. The magnitude and composition of emergent taxa varied throughout the sampling period, with sequential peaks in biomass that altered the prey available to riparian consumers. Our results suggest that tidal freshwaters export substantial quantities of aquatic insects, which are valuable prey items for riparian consumers in these systems.