Testing for thresholds of ecosystem collapse in seagrass meadows

Abstract

Although the public desire for healthy environments is clear-cut, the science and management of ecosystem health has not been as simple. Ecological systems can be dynamic and can shift abruptly from one ecosystem state to another. Such unpredictable shifts result when ecological thresholds are crossed; that is, small cumulative increases in an environmental stressor drive a much greater change than could be predicted from linear effects, suggesting an unforeseen tipping point is crossed. In coastal waters, broad-scale seagrass loss often occurs as a sudden event associated with human-driven nutrient enrichment (eutrophication). We tested whether the response of seagrass ecosystems to coastal nutrient enrichment is subject to a threshold effect. We exposed seagrass plots to different levels of nutrient enrichment (dissolved inorganic nitrogen) for 10 months and measured net production. Seagrass response exhibited a threshold pattern when nutrient enrichment exceeded moderate levels: there was an abrupt and large shift from positive to negative net leaf production (from approximately 0.04 leaf production to 0.02 leaf loss per day). Epiphyte load also increased as nutrient enrichment increased, which may have driven the shift in leaf production. Inadvertently crossing such thresholds, as can occur through ineffective management of land-derived inputs such as wastewater and stormwater runoff along urbanized coasts, may account for the widely observed sudden loss of seagrass meadows. Identification of tipping points may improve not only adaptive-management monitoring that seeks to avoid threshold effects, but also restoration approaches in systems that have crossed them.

Biodiversity and ecosystem services in Nordic coastal ecosystems

“…When nutrient enrichment exceeded moderate levels, a switch from positive to negative net leaf production was observed. Epiphyte load also increased with nutrient enrichment, potentially driving this shift (Connell et al, 2017). Eventually, it may cause eelgrass meadows to decrease.…”

“…The reasons for this decline are construction of e.g. marinas (Moksnes et al, 2016) and eutrophication, which leads to shading by epiphytes and poor oxygen condition in the sediment (Connell et al, 2017), along with lower abundance of predatory fish. Some efforts are now being made to restore eelgrass meadows.…”

Environmental solutions sparked by environmental history

“…Nitrogen targets remain subject of intensive research into identifying the thresholds of nutrient-driven loss (Connell et al 2017;Fernandes et al 2019). The ecological baselines, which increase certainty in investment planning, are used to design environmental improvement Conservation Biology Volume 34, No.…”

“…Nitrogen targets remain subject of intensive research into identifying the thresholds of nutrient‐driven loss (Connell et al. ; Fernandes et al. ).…”

Biodiversity and ecosystem services in Nordic coastal ecosystems

“…The response of seagrass ecosystems to coastal nutrient enrichment has shown to follow a "threshold pattern" when nutrient enrichment exceeded moderate levels, with a switch from positive to negative net leaf production. This shift is potentially driven by increased epiphyte load due to nutrient enrichment, which blocks light and reduces health of the seagrass leaves (Connell et al, 2017). Effective management of land-derived nutrient inputs from e.g.…”