Agricultural pressures impair trophic link between aquatic microorganisms and invertebrates

Decadal declines in aquatic ecosystem health prompted monitoring efforts and studies on effects of human practices on aquatic biodiversity, yet a consideration of ecological processes and trophic linkages is increasingly required to develop an in-depth understanding of aquatic food webs and its vulnerability to human activities. Here, we test in laboratory incubations using natural organic matter whether agricultural practices have an effect on two interacting ecological processes (i.e., decomposition and invertebrate growth) as the relevant temporal components of the trophic linkage between aquatic microbial communities and aquatic invertebrates. We further assess whether these altered trophic interactions are visible on ecologically relevant scales. We observed clear patterns in agricultural constraints on microbial decomposition, which coincided with reduced invertebrate growth and an unexpected increase in invertebrate consumption of organic matter. Similar differences in invertebrate length depending on land use were observed in our field survey, thereby providing important clues on the relevance and vulnerability of interdependent processes that can serve to improve future forays in monitoring ecosystem health.

Agricultural practices are a primary source of pollution to aquatic sytems. This is due to 61 the use of chemicals such as pesticides and fertilizer that run-off to adjacent aquatic systems 62 where they can have direct-or indirect toxic effects in the water column or associate with OM 63 accumulating in their sediments (e.g., Knezovich et al., 1987;Vijver et al., 2017 decomposition. This is relevant since the process of microbial decomposition, in contrast to 76 microbial diversity, provides a more reliable reflection of OM-conditioning and available 77 microbial biomass, and therewith the resources available for invertebrates, for the time period 78 (seasons to years) that is required for OM to decompose and invertebrates to develop (e.g.,    Asellus aquaticus (being a common detritivore in many freshwater habitats) on a diet of OM 116 collected from ditches adjacent to either flower bulb fields, grasslands or pristine dune area. A. 117 aquaticus was chosen as a model organism as it is a highly abundant detritivore in aquatic ditch 118 systems. We prepared Decomposition and Consumption Tablets (DECOTABs) as standardized 119 OM substrate as described by Kampfraath et al. (2012) and Van der Lee (2020). In brief, 120 DECOTABs (ø17 mm) were prepared from naturally-derived OM embedded in an agar matrix,  addition, DECOTABs which were only exposed to substrate-dependent microbial 150 decomposition were dried and weighed following the same protocol. In order to assess initial 151 DECOTAB weight, ten unused DECOTABs per treatment were dried and weighed following 152 the same. Thereafter, both microbial decomposition and isopod consumption of OM was 153 analyzed using a one way-ANOVA with a Tukey HSD post-hoc test.

174
Ecoplates are comprised of ecologically relevant, structurally diverse compounds, yet do not 175 include e.g., recalcitrant substrates nor specific substrates typical of the soils used in this study.

176
It is therefore impossible to directly relate substrate utilization profiles to the actual functioning 177 of the soil microbial communities. Nonetheless, the number of substrates used can serve as a 178 proxy of the metabolic diversity of the microbial community (Garland, 1999). To this end, 179 microbial communities were sampled from sediments adjacent to dunes, grasslands, and bulb 180 fields (6 replicates per treatment). One mL of sediment was diluted 50 times with demineralized 181 water and vortexed. Mineral substrate was allowed to settle and subsequently distributed over

188
After 42 days of incubation, Asellus aquaticus contributed significantly (ANOVA, F = 189 56.8, p < 0.001) more to DECOTAB mass loss, when compared to DECOTAB exposed solely 190 9 to microbial biofilms (Fig 1A). A shift in relative contributions to OM degradation becomes 191 evident when plotting the ratio of microbial decomposition and isopod-associated OM 192 consumption (Fig 1B), illustrating a shift towards a higher contribution of microbial 193 decomposition to OM degradation. Isopods contributed to 37.8% of biomass loss of dune-194 derived OM, whereas they contributed to 85.2% and 90.3% of biomass loss in bulb field-and 195 grassland-derived OM, respectively. We observed major differences between the ratios of

205
Over the course of 42 days, we observed that A. aquaticus feeding on OM derived from 206 the dune area had significantly higher growth rates compared to A. aquaticus that fed on 207 DECOTABs with grass-or bulb OM (TukeyHSD: pdune-bulb < 0.001 and pdune-grass < 0.001), 208 whereas no difference in growth rates was observed between A. aquaticus that fed on bulb-and 209 grass OM (TukeyHSD: p = 0.377; Fig. 2). In addition, A. aquaticus that fed on grassland-      forms a key source of fine particulate OM for filter-and deposit feeding invertebrates 304 (Bundschuh & McKie, 2016). Impaired invertebrate growth rates may also lead to reduced 305 biomass available for predators that rely on invertebrate shredders as food source, such as fish 306 (Rask & Hiisivuori, 1985) and invertebrate predators (Herrmann, 1984; Krisp & Maier, 2005).

307
In other words, current monitoring efforts that rely on single time point estimation of 308 invertebrate abundances fail to capture the temporal nature and complexity of trophic 309 interactions (e.g., knock-on effects on growth rate and other fitness variables) that ultimately 310 govern the functioning and health of ecosystems. Thus, the likelihood that monitoring efforts 311 can lead to misinterpretations of ecosystem health assessments calls for reconsidering our 312 approaches in biodiversity assessments and environmental diagnostics.