Dredging fundamentally reshapes the ecological significance of 3D terrain features for fish in estuarine seascapes

Context: Landscape modification alters the condition of ecosystems and the structure of terrain, with widespread impacts on biodiversity and ecosystem functioning. Seafloor dredging impacts a diversity of flora and fauna in many coastal landscapes, and these processes also transform three-dimensional terrain features. The potential ecological significance of these terrain changes in urban seascapes has, however, not been investigated. Objectives: We examined the effects of terrain variation on fish assemblages in 29 estuaries in eastern Australia, and tested whether dredging changes how fish associate with terrain features. Methods: We surveyed fish assemblages with baited remote underwater video stations and quantified terrain variation with nine complementary metrics (e.g. depth, aspect, curvature, slope, roughness), extracted from bathymetry maps created with multi-beam sonar. Results: Fish diversity and abundance were strongly linked to seafloor terrain in both natural and dredged estuaries, and were highest in shallow waters and near features with high curvature. Dredging, however, significantly altered the terrain of dredged estuaries and transformed the significance of terrain features for fish assemblages. Abundance and diversity switched from being correlated with lower roughness and steeper slopes in natural estuaries to being linked to features with higher roughness and gentler slopes in dredged estuaries. Conclusions: Contrasting fish-terrain relationships highlight previously unrecognised ecological impacts of dredging, but indicate that plasticity in terrain use might be characteristic of assemblages in urban landscapes. Incorporating terrain features into spatial conservation planning might help to improve management outcomes, but we suggest that different approaches would be needed in natural and modified landscapes

Seafloor Terrain Shapes the Three-dimensional Nursery Value of Mangrove and Seagrass Habitats

Mangroves and seagrasses are important nurseries for many marine species, and this function is linked to the complexity and context of these habitats in coastal seascapes. It is also connected to bathymetric features that influence habitat availability, and the accessibility of refuge habitats, but the significance of terrain variation for nursery function is unknown. To test whether seafloor terrain influences nursery function, we surveyed fish assemblages from mangrove and seagrass habitats in 29 estuaries in eastern Australia with unbaited underwater cameras and quantified the surrounding three-dimensional terrain with a set of complementary surface metrics (that is, depth, aspect, curvature, slope, roughness) applied to sonar-derived bathymetric maps. Terrain metrics explained variability in assemblages in both mangroves and seagrasses, with differing effects for the entire fish assemblage and nursery species composition, and between habitats. Higher depth, plan curvature (concavity or convexity) and roughness (backscatter) were negatively correlated with abundance and diversity in mangroves and positively linked to abundance and diversity in seagrass. Mangrove nursery species (6 species) were most abundant in forests adjacent to flats with concave holes, rough substrates and low-moderate depths, whereas seagrass nursery species (3 species) were most abundant in meadows adjacent to deep channels with soft mounds and ledges. These findings indicate that seafloor terrain influences nursery function and demonstrate contrasting effects of terrain variation in mangroves and seagrass. We suggest that incorporating three-dimensional terrain into coastal conservation and restoration plans could help to improve outcomes for fisheries management, but contrasting strategies might be needed for different nursery habitats

Dredging transforms the seafloor and enhances functional diversity in urban seascapes

Landscape modification alters the condition of ecosystems and the complexity of terrain, with consequences for animal assemblages and ecosystem functioning. In coastal seascapes, dredging is routine practice for extracting sediments and maintaining navigation channels worldwide. Dredging modifies processes and assemblages by favouring species with wide trophic niches, diverse habitat requirements and tolerances to dredge-related eutrophication and sedimentation. Dredging also transforms the three-dimensional features of the seafloor, but the functional consequences of these terrain changes remain unclear. We investigated the effects of terrain modification on the functional diversity of fish assemblages in natural and dredged estuaries to examine whether dredging programs could be optimised to minimise impacts on ecological functioning. Fish assemblages were surveyed with baited remote underwater video stations and variation in functional niche space was described using species traits to calculate metrics that index functional diversity. Terrain variation was quantified with nine complementary surface metrics including depth, aspect, curvature, slope and roughness extracted from sonar-derived bathymetry maps. Functional diversity was, surprisingly, higher in dredged estuaries, which supported more generalist species with wider functional niches, and from lower trophic levels, than natural estuaries. These positive effects of dredging on functional diversity were, however, spatially restricted and were linked to both the area and orientation of terrain modification. Functional diversity was highest in urban estuaries where dredged channels were small (i.e. <1% of the estuary), and where channel slopes were orientated towards the poles (i.e. 171-189 degrees), promoting both terrain variation and light penetration in urban estuaries. Our findings highlight previously unrecognised functional consequences of terrain modification that can easily be incorporated into dredging programs. We demonstrate that restricting the spatial extent of dredging operations and the orientation of dredged channel slopes, wherever this is practical, could help to limit impacts on ecosystem functioning and productivity in urban seascapes

Landscape transformation alters functional diversity in coastal seascapes

Flint, N ORCiD: 0000-0003-4331-4109Ecography published by John Wiley & Sons on behalf of Nordic Society Oikos The ecological impacts of landscape modification and urbanisation have transformed the composition of plant and animal assemblages, and altered the condition of ecosystems globally. Landscape transformation influences the spatial distribution of species and ecological functions by selecting for generalist species with wide ecological niches, which can adapt to opportunities in highly-modified environments. These effects of landscape modification can shape functional diversity on land, but it is not clear whether they have similar functional consequences in the sea. We used estuaries as a model system to test how landscape transformation alters functional diversity in coastal seascapes, and measured how variation in level of urbanisation, catchment modification and habitat loss influenced fish diversity across thirty-nine estuaries in eastern Australia. Fish were surveyed with baited remote underwater video stations and functional diversity was indexed with three metrics that describe variation in the functional traits and niche space of assemblages. The extent of landscape transformation in the catchment of each estuary was associated with variation in the functional diversity of estuarine fish assemblages. These effects were, however, not what we expected as functional diversity was highest in modified estuaries that supported a large area of both urban and grazing land in their catchments, were bordered by a small area of natural terrestrial vegetation and that contained a moderate area of mangroves. Zoobenthivores and omnivores dominated assemblages in highly-modified estuaries, and piscivorous fishes were common in natural waterways. Our results demonstrate, that the modification and urbanisation of ecosystems on land can alter functional diversity in the sea. Intense landscape transformation appears to select for abundant generalists with wide trophic niches, and against species with specialised diets, and we suggest that these changes might have fundamental consequences for ecosystem functioning in estuaries, and other highly modified seascapes. © 2019 The Authors