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.

Figure 1

(a) Localisation of the Bay of Brest within the French Atlantic coast and of the study area. (b) Dredging intensities (Pressure) calculated over 5 years for 50 × 50 m squares within the study area together with localisation of the maerl beds (bordered with gray lines). Black stars represent stations sampled for SPI images only (black stars) and white stars the stations sampled for both SPI images and cores. Maps were created using QGIS 2.18.1 software (QGIS Development Team. QGIS Geographic Information System. Open Source Geospatial Foundation Project. http://qgis.osgeo.org).

Figure 2

Photographs showing examples of cross-section images of maerl beds obtained with SPI in stations belonging to the Control (a), moderate (b) and High (c) dredging intensity groups and corresponding semi-automatically analyzed images (d–f). Red lines refer to the water-maerl interface, yellow and blue overlays to live and dead maerl, respectively.

Figure 3

Dredging intensities (summed over 5 years) measured in the 30 sampled stations. Black circles, white squares and black triangles indicate stations belonging to the Control, Moderate and High dredging intensity groups, respectively.

Figure 4

Relationships between dredging intensity and station-scale averages/standard deviations of Interface rugosity (a,b), SPI penetration depth (c,d) and maerl live/dead ratios (e,f).

Figure 5

Vertical profiles of mean (±SE) station-scale averages/standard deviations of the relative proportions occupied by live maerl, dead maerl and Interstitial space in the three dredging intensity groups. Black circles, white squares and black triangles indicate Control, Moderate and High dredging intensity groups, respectively.

Figure 6

Relationships between dredging intensity and station-scale averages (+/− SD) of maerl thalli mean perimeter (a) and maerl thalli mean solidity index (b).