Dry conditions disrupt terrestrial-aquatic linkages in northern catchments

Abstract

Aquatic ecosystems depend on terrestrial organic matter (tOM) to regulate many functions, such as food web production and water quality, but an increasing frequency and intensity of drought across northern ecosystems is threatening to disrupt this important connection. Dry conditions reduce tOM export and can also oxidize wetland soils and release stored contaminants into stream flow after rainfall. Here, we test whether these disruptions to terrestrial-aquatic linkages occur during mild summer drought and whether this affects biota across 43 littoral zone sites in 11 lakes. We use copper (Cu) and nickel (Ni) as representative contaminants, and measure abundances of Hyalella azteca, a widespread indicator of ecosystem condition and food web production. We found that tOM concentrations were reduced but correlations with organic soils (wetlands and riparian forests) persisted during mild drought and were sufficient to suppress labile Cu concentrations. Wetlands, however, also became a source of labile Ni to littoral zones, which was linked to reduced abundances of the amphipod H. azteca, on average by up to 70 times across the range of observed Ni concentrations. This reveals a duality in the functional linkage of organic soils to aquatic ecosystems whereby they can help buffer the effects of hydrologic disconnection between catchments and lakes but at the cost of biogeochemical changes that release stored contaminants. As evidence of the toxicity of trace contaminant concentrations and their global dispersion grows, sustaining links among forests, organic soils and aquatic ecosystems in a changing climate will become increasingly important.

Keywords: Hyalella azteca; climate change; dissolved organic carbon; drought; littoral invertebrates; metal toxicity; organic soils; terrestrial organic matter.

Figure 1

Drought reduces dissolved organic carbon (DOC) export but elevates metal release in catchments with high wetland influence. DOC concentrations increase minimally from the low wetland (1% wetland cover) catchment (a) but decrease from the high wetland (21% wetland cover) catchment (b). Labile metal concentrations (Ni_L, Cu_L) show minimal change from the low wetland catchment (c, e) vs. large increases from the high wetland catchment (d, f). Open circles in panel (b) indicate an increase in DOC observed during the onset of drought.

Figure 2

Disrupted terrestrial–aquatic linkages during mild drought conditions. Terrestrial organic matter (tOM) concentration dissolved organic carbon (DOC) increases with total wetland (a) and riparian forest density (b) and in turn suppresses labile Cu concentration (d). Labile Ni increases with connected wetland area (c), which in turn suppresses Hyalella azteca abundances (e). Lines indicate mean model fit ±95% CI (grey polygons). Points are (a–d) site‐ or (e) dendy‐level partial residuals. Model statistics are reported in Table 1.

Figure 3

Path analysis of linkages between terrestrial organic soils and aquatic biota (Hyalella azteca abundances), as disrupted by drought. Model effect sizes are shown for the two linkages: (L1) the provision of terrestrial organic matter (tOM; measured as DOC) as a regulator of contaminant lability, and (L2) the supply of labile metal contaminants from organic soils. Wetland area is either total wetland influence (in L1 models) or connected wetland area (in L2 models). Parameters with no significant relationships are greyed. Model statistics and parameters are reported in Table 1.