Capturing threshold responses of marine benthos along gradients of natural and anthropogenic change

Abstract

Ecologists and managers need to understand what types of communities emerge with continued human alterations to ecosystems against a background of natural change. Both natural and anthropogenic drivers are well known to affect organisms’ distributions; however, it often remains unclear where along a range of environmental and anthropogenic gradients important compositional community changes occur. We used a big‐data approach, including over 175,000 presence records of benthic genera for the North Sea, to identify environmental (bed shear stress, sediment grain size, temperature) and anthropogenic parameters (trawling effort) driving benthic community composition over a 21‐year period. We applied a Gradient Forest analysis, based on Random Forests, to estimate the locations and importance of thresholds where small cumulative increases in the predictors drive a much greater change in genus composition than would be expected from linear effects. Shear stress was the most important predictor of benthic community composition. Trawling effort, temperature gradients and sediment grain size were of intermediate importance. This corroborates that current and wave effects (typically associated with seabed substrate types) are primary determinants of benthic communities. Our results suggest that a genus composition threshold for both infauna and epifaunal benthic communities is crossed when the seafloor is trawled as little as once every 4 years. Higher trawling levels corresponded with gradual compositional change without obvious thresholds, which would be consistent with chronic fishing in the North Sea over the last two centuries having caused persistent, long‐term changes in ecosystem structure and functioning. This was corroborated by the large‐scale spatial patterns of benthic community composition undergoing limited temporal changes during the 21‐year study period. Synthesis and applications. Although well established in theory, threshold effects are poorly validated in the field. We generated new information on multi‐organism responses to environmental change at the scale of a continental shelf ecosystem and over a multi‐decadal time period. This will help pure and applied scientists better understand the conditions under which community thresholds are crossed and provide environmental managers with empirical evidence that is expected to reduce uncertainty regarding decisions on the protection and sustainable use of the marine environment.

What’s hot and what’s not: Making sense of biodiversity ‘hotspots’

“…Like our study, Couce, Engelhard, et al. (2020) found shear stress to be the primary driver of change across North Sea marine benthic assemblages. However, they showed that beam and otter trawling had pronounced effects with clear thresholds of change on specific benthic genera that were not evident at the assemblage level in our assessment.…”

“…It would be useful in future therefore to specifically select sites where temporal trawl and survey data correspond and apply the threshold approach after Couce, Engelhard, et al. (2020) to the more extensive dataset presented here. Such an approach could reveal where along anthropogenic gradients such biotic thresholds exist and thus gauge where pressures become unsustainable.…”

“…Using the same spatial grid as for diversity estimation, we collated environmental variables considered important determinants of benthic assemblage structure (e.g. Barrio Froján et al., 2012; Cooper & Barry, 2017; Couce, Engelhard, et al., 2020; Davies et al., 2004; Renaud et al., 2009), including depth, sediment composition, bed shear stress, temperature, salinity, nitrate, phosphate, dissolved oxygen, chlorophyll, bottom current speed and particulate organic carbon. We also included data on fishing pressure (hours fishing per year) derived from an analysis of Vessel Monitoring System (VMS) data for UK vessels larger than 15 m and segregated by gear type.…”

Trawl fishing impacts on the status of seabed fauna in diverse regions of the globe

“…What are the cascading effects of such a status on the wider marine ecosystem? Reference points for benthic invertebrates are undeveloped and will require further research to determine them, which will likely be specific to a given region (Couce, Engelhard, & Schratzberger, 2019; Lambert et al., 2017). However, the specificity of the status information provides useful quantitative guidance for implementing management measures to mitigate the impacts (McConnaughey et al., 2020).…”

Long non-coding RNA LncMPEG1 responds to multiple environmental stressors by affecting biomineralization in pearl oyster Pinctada fucata martensii

“…Marine benthic communities are subjected to changing environmental conditions (Couce et al, 2020). Mollusca is the second most bio-diverse extant molluscan phylum, in which shelled mollusks are distinguished by a vast array of biomineralization fabricated architectures (Williams, 2007).…”