Aquatic macrophytes alter productivity‐richness relationships in eutrophic stream food webs

Abstract

Abstract. Traditional productivity-diversity theory predicts that eutrophication will result in greater species richness due to increased resources at the bottom of the food web. However, few studies on the effects of increasing ecosystem productivity on biological communities have included responses at multiple trophic levels. We hypothesized that the effect of eutrophication on species richness would vary between different trophic levels due to shifts in community composition and trophic interactions. To investigate the mechanisms driving productivity-richness relationships, we constructed food webs for 18 streams across a eutrophication gradient on South Island, New Zealand, and measured productivity, water quality, and habitat characteristics in each stream. A principal components analysis yielded two orthogonal axes of eutrophication: one associated with macrophytes, benthic substrate, and gross primary productivity (GPP) and the other with catchment area, temperature, and algal standing stock (chlorophyll a). Surprisingly, the majority of community response variables, especially defended primary consumer abundance and biomass, were more strongly associated with the macrophyte/habitat axis. The lack of change in abundance and declines in biomass and average mass of predatory invertebrates and fish with increasing eutrophication, despite increases in prey abundance, suggest that energy was not being passed up the food chain to higher trophic levels. The predominance of defended producers (macrophytes) and consumers (snails and microcrustacea) in eutrophic streams likely served as trophic bottlenecks; both are largely inedible by higher trophic levels. Consequently, our results suggest that managers need to focus on preserving food web linkages and energy flow, as well as biodiversity, in eutrophic systems.

Leaf litter additions enhance stream metabolism, denitrification, and restoration prospects for agricultural catchments

“…, Graham et al. ) or inputs from the existing riparian grasses. While grasses, aquatic macrophytes, and autotrophic biofilms may provide a significant carbon source (Menninger and Palmer , Burrell et al.…”

“…Current riparian zones along agricultural waterways in the Canterbury Plains either lack riparian vegetation or exist primarily as grass alone (Wilcock et al 2009, Renouf andHarding 2015). As a result, organic carbon sources in agricultural waterways in the region tend to be dominated by autochthonous production (Greenwood et al 2012, Burrell et al 2014, Graham et al 2015 or inputs from the existing riparian grasses. While grasses, aquatic macrophytes, and autotrophic biofilms may provide a significant carbon source (Menninger andPalmer 2007, Burrell et al 2014), they are often quick to decompose, so a goal for riparian plantings aimed at boosting in-stream denitrification should be to provide sources of carbon which are consistently available.…”

The influence of pastoral and native forest land cover, flooding disturbance, and stream size on the trophic ecology of New Zealand streams

“…Finally, pastoral cover also did not affect either armoured or unarmoured invertebrate biomass, despite conditions for armoured invertebrates often being optimised in agriculturally influenced streams (i.e. slow water velocity and high sediment input; Wootton et al 1996;Graham et al 2015) while unarmoured taxa usually suffer population declines because they are more sensitive to poor water quality and lack of streambed interstices (Quinn & Hickey 1990).…”

“…Unarmoured invertebrates such as Ephemeroptera and Plecoptera are palatable and are frequently preyed on by fishes, potentially increasing small-bodied fish biomass, which are in-turn consumed by piscivorous fishes. Armoured invertebrates such as Gastropoda and cased Trichoptera are less-palatable for fishes (Wootton et al 1996) and thus may be negatively correlated with fish biomass (Graham et al 2015;Jellyman & McIntosh 2020).…”

Combining Tools from Edge-of-Field to In-Stream to Attenuate Reactive Nitrogen along Small Agricultural Waterways

“…Nutrient pollution from agricultural land use has degraded water quality and aquatic ecosystem health, creating significant management challenges for aquatic ecosystems around the world [1,2]. Excess reactive nitrogen (N) and phosphorus (P) inputs can cause eutrophication, toxic algal blooms, anoxic dead zones, altered food webs in receiving freshwater and estuarine environments, and nitrate toxicity in groundwater [2][3][4]. Small agricultural streams and ditches are the beginning of drainage systems that receive and transport excess nutrients to larger downstream waterways [5,6].…”