Effects of seasonality and resource limitation on organic matter turnover by Chironomidae (Diptera) in southern Appalachian headwater streams

Abstract

Despite their high abundance, secondary production, and known reliance on detrital material, the role of chironomids (Diptera) in fine particulate organic matter (FPOM) dynamics has not been well quantified. We conducted field trials using fluorescent pigment markers to estimate seasonal rates of consumption, annual secondary production, assimilation efficiency (AE), and FPOM turnover for non‐Tanypodinae chironomids in two similar, southern Appalachian, headwater streams. An ecosystem‐level experimental manipulation of detrital inputs in one study stream allowed for a unique assessment of turnover in reference, resource‐limited, and resource‐recovery systems. Seasonality had a significant effect on larval gut‐passage time (GPT), with faster GPT at higher temperatures. Mean GPT ranged from 39 min (summer) to 67 min (winter). Mean annual FPOM turnover was reduced in the resource‐limited stream (5.5 kg ash‐free dry weight per year; 12% of mean annual FPOM export) compared to the reference stream (46 kg yr−1; 74% of mean export). Following 4 yr of resource recovery, FPOM turnover increased to 24 kg ash‐free dry wt yr−1 (16% of export) but remained lower than levels measured in the reference stream (53 kg ash‐free dry wt yr−1; 20% of export). Chironomid contribution to FPOM turnover was much higher in low vs. high discharge conditions (74% in low discharge vs. 20% in high discharge). Assimilation efficiency was estimated to be low in all systems (1.7–2.5%). Chironomids turned over a large portion of FPOM available to them, suggesting that turnover by the entire collector community in forested headwater systems may be much higher than expected.