New Aquatic Insect Emergence Records From Streams of the Hubbard Brook Experimental Forest Reveal Both Spatial and Interannual Variation in Insect Emergence

Aquatic insect emergence is a critical life history stage whose magnitude and phenology respond to multiple environmental factors. Here, we report the first estimates of spatial and temporal variation in aquatic insect emergence from oligotrophic streams of the Hubbard Brook Experimental Forest (HBEF, New Hampshire, USA) over the period 2018–2024. This record is one of very few multi‐site, multi‐year aquatic insect emergence time series.

We measured insect emergence from seven gauged, headwater streams and the mainstem of Hubbard Brook, a fifth‐order stream into which all of the headwater streams drain. Emergence traps were deployed and collected weekly during the snow‐free months (April—November). These emergence samples complement long‐term weekly stream chemistry monitoring (since 1963) and were coincident with high‐frequency water temperature monitoring. Insects were collected on sticky traps hung above the streams and identified to order level. Here, we report the emergence patterns for two streams over 7 years, five streams for 3 years, and one stream for 2 years.

Total annual emergence counts of aquatic insects varied from the lowest count of 1887 individuals from the Hubbard Brook mainstem in 2018 to 22,628 individuals from headwater stream W5 in 2022. Emergence was dominated by aquatic Diptera, and despite the large variation in emergence magnitude, the timing of peak emergence was consistently in mid to late May (Julian day = 140 ± 9).

Our data document highly variable secondary productivity from these streams despite their low nutrient availability and strong seasonality in peak emergence timing.

Collection of emergence samples is ongoing which—when coupled with long‐term HBEF streamflow and chemistry data—represents an opportunity to examine the effects of increasingly intense precipitation events and resulting high streamflow, which may scour the streambed, as well as the effects of recovery from acid rain, which is increasing stream pH. Understanding the consequences of changing stream temperature, flow, and chemistry on the magnitude and timing of aquatic insect emergence is critical for predicting effects on their populations and life histories.