Flow Reduction in a Pesticide-Exposed Stream Mesocosm Affects Emerging Aquatic Insects and Alters Riparian Spider Communities

Abstract

Water scarcity can intensify streamflow reduction, alter hydromorphology, increase chemical pollution, and disrupt resource exchange between aquatic and terrestrial ecosystems. However, the effects of streamflow reduction on pesticide concentrations in sediment, and how these changes influence aquatic insect emergence and riparian spider communities, remain poorly understood. We conducted a 39-day mesocosm experiment in Southwest Germany using 12 artificial stream mesocosm with adjacent riparian areas, randomly assigned to low-flow (0.4 L s^-1) treatment or control (1 L s^-1) to simulate flow alteration. We sampled water daily (47 days), sediment (weeks 4 and 6), emerging insects (weekly for 5 weeks), and riparian spiders (week 6). Our results show that under low-flow conditions, the mean sediment pesticide concentrations increased slightly stronger from week 4 to week 6, compared to the control. However, the high variability within treatments resulted in no significant differences between the treatments. Low flow caused a 2.9-fold reduction in the total abundance of Ephemeroptera, Plecoptera, and Trichoptera (EPT), while total emerging insects (abundance and biomass) and spider abundance were not affected significantly. However, the web-building spider Tetragnatha sp. was 2.6-fold less abundant under low-flow conditions, likely due to vegetation-habitat-related responses and reduced EPT abundance, while the ground-hunting Lycosidae were unaffected, owing to their active hunting behavior and generalist strategy. These findings highlight the need to consider cross-ecosystem effects when assessing the ecological impacts of hydrological alterations in contaminated aquatic ecosystems.

Fig. 1

Experimental setup of RSM. a Schematic of an RSM unit, showing the flume and adjacent riparian area. The emergence trap was placed partially on the gravel bank to reflect habitat heterogeneity and positioned at either the inlet, middle, or outlet section within each flume. Solid outlines indicate the trap position in each flume, while dashed outline represent alternative positions used in other flumes, illustrating placement variation across flumes. b Diagram of random allocation of flume to individual treatments across the twelve RSM units; units 3, 8, 12, and 15 were intentionally excluded from the experiment

Fig. 2

Time series plot of the median daily pesticide concentrations in water supplying the RSM. Rectangles represent fungicides, triangles represent herbicides, and asterisks represent insecticides. The vertical line “start of low-flow” indicates the beginning of the treatment phase in the flumes

Fig. 3

Mean sediment pesticide concentrations (n = 6) in control and low-flow treatments across two sampling weeks, with error bars representing the 95% confidence interval (CI). Circles (solid lines) represent week 4 samples, while triangles (dashed lines) represent week 6 samples in both the control and low-flow treatment

Fig. 4

a Mean abundance (± standard error [SE]; n = 6) of emerging aquatic insects in the control (circles, solid lines) and low-flow treatments (triangles, dashed lines) across five sampling weeks. b Mean biomass (± standard error [SE]; n = 6) of emerging aquatic insects in the control and low-flow treatments across five sampling weeks

Fig. 5

Mean abundance (n = 6) of two spider taxa in the control (circles, solid lines) and low-flow (triangles, dashed lines) treatments on day 39, with error bars representing the standard error (SE). Letters above the bars indicate significant differences (emmeans: p < 0.05)