Declining maerl vitality and habitat complexity across a dredging gradient: Insights from in situ sediment profile imagery (SPI)

Abstract

Maerl beds form complex biogenic benthic habitats, characterized by high productivity as well as diverse biological communities. Disturbances associated with extraction and/or fishing activities using mobile bottom-contacting gears such as clam-dredges induce the most severe and long-term effects on these fragile habitats. We here investigated the effects of dredge-fishing on maerl in the bay of Brest (France). We quantified maerl beds structure and vitality across a fine scale quantified dredging intensity gradient through the acquisition of in-situ images of beds cross-section using Sediment Profile Imaging system (SPI). Declines in the proxies of maerl vitality and habitat complexity were measured across the gradient, and were associated with significant changes in the vertical distribution of live and dead maerl as well as of interstitial space. Fishing with dredges caused maerl mortality, substratum compaction, and decreasing habitat complexity. SPI imaging techniques also allowed for an assessment of changes in spatial heterogeneity that dredging created on several aspects of the structure and vitality of maerl beds. It suggests that direct and indirect disturbances induced by dredging are not acting at the same spatial scale, and can thereby differentially affect the ecosystem functions linked to vitality and habitat complexity.

Bottom Trawling Threatens Future Climate Refugia of Rhodoliths Globally

“…Despite their ecological, conservational and evolutionary importance, refugia are currently under great local stress by intense bottom trawling, reaching up to ∼140,000 days, annually. Such localized and intense disturbances, acting in synergy with ongoing and projected climate changes, are likely to compromise the fate of the whole biome and its associated communities, since long lasting or irreversible impacts have already been observed in previously dredged regions, on the scale of years post-dredging (Hall-Spencer and Moore, 2000;Bernard et al, 2019).…”

“…Rhodolith refugia are under great local threat from intensive bottom trawling (∼79,000-114,000 days annually), requiring urgent conservation management measures and regulation of this activity. Bottom trawling causes massive mortality of rhodoliths and their associated organisms, as it crushes, buries and eliminates any natural bottom features, while the plume of sediments lifted by trawling reduces light availability, blocks photosynthesis and contributes to additional losses of nearby, non-trawled rhodoliths (Hall-Spencer and Moore, 2000;Thrush and Dayton, 2002;Steller et al, 2003;Wilson et al, 2004;Gabara et al, 2018;Bernard et al, 2019). Such profound impacts are mostly irreversible and damages can still be observed for years post-trawling (Hall-Spencer and Moore, 2000; but see Barberá et al, 2017;Ordines et al, 2017).…”

“…Additionally, they are important carbonate factories, contributing to the global carbon cycling (Amado-Filho et al, 2012a;van der Heijden and Kamenos, 2015). Although their ecological value has been acknowledged since the 19th century (Gran, 1893;Pruvot, 1897), rhodolith habitats are globally decreasing, mostly as a result of bottom fishing disturbance, and are expected to continue to decrease under the upcoming environmental changes, especially ocean warming and acidification (Hall-Spencer and Moore, 2000;Noisette et al, 2013;Martin and Hall-Spencer, 2017;Bernard et al, 2019). Anticipating and reducing the cumulative impacts on these rich and important ecosystems could improve their resilience, and in turn, promote and guide well-informed conservation and management of marine biodiversity.…”

Variation in Photosynthetic Performance Relative to Thallus Microhabitat Heterogeneity in Lithothamnion australe (Rhodophyta, Corallinales) Rhodoliths

“…The fragmentation of large individuals into smaller individuals can therefore result in the loss of structural complexity (Hall‐Spencer and Moore 2000), which in turn reduces overall photosynthetic rates (Bernard et al. 2019) and removes interstitial spaces that support microinvertebrates and trap detritus. Here, we define "microhabitats" as different parts of the rhodolith thalli that are associated with their interior and exterior branches, and their interbranch spaces.…”

Cryptofauna associated with rhodoliths: Diversity is species‐specific and influenced by habitat

“…Studies have shown that structural damage to rhodolith beds can result in significantly reduced richness of invertebrates, and reduced abundance of macroalgae, invertebrates and fish, with implications for other mobile marine species that make use of the secondary productivity associated with rhodoliths (e.g., Gabara et al, 2018;Tompkins & Steller, 2016). Bernard et al (2019) found that substratum compaction, increased rhodolith mortality and decreasing habitat complexity occurred across a gradient of increasing dredging intensity.…”