Sampling native and non-native mobile epifauna with baited traps and habitat collectors – Port of Gdynia case study (southern Baltic Sea, Poland)

The effectiveness of two artificial habitat collectors, crab condo (HC1) and habitat crate (HC2), providing a refuge for small mobile fauna, was tested along with two commercial baited traps, Chinese box trap (BT1) and Gee’s Minnow trap (BT2) recommended for only single deployments under a harmonized survey of the Baltic and the North-East Atlantic. Our objective was also to determine whether a multi-deployment of baited traps in the growing season increases the diversity and abundance of collected mobile epifauna. Nineteen species of benthic mobile epifauna, including six non-indigenous species (NIS), were collected between May and October 2014 using all tested types of traps in the Port of Gdynia (southern Baltic Sea). Crustaceans, represented by 16 taxa, constituted the group with the highest diversity and abundance. Our study showed that HC1 and HC2 are more effective gear than BT1 and BT2, as both species richness (including NIS) and abundance were higher. Furthermore, the double deployment of BT1 and BT2 increased the diversity and abundance of the captured fauna. The use of artificial habitat collectors as an additional method to the already recommended baited traps for mobile epifauna monitoring in ports should be considered and the number of baited trap deployments should be increased during the growing season.

Cover letter RE “Video observations of the acute effects of dredged sediment deposition on mobile epifauna”

The “beneficial uses” of dredged sediment are increasingly being explored for habitat restoration and beach nourishment. At ocean and nearshore deposition sites, any beneficial use must be tempered by evaluating impacts to the benthos. We studied a site at the mouth of the Columbia River where a “thin-layer” sediment deposition method was employed to minimize mounding and disperse sediment within a prescribed area. We used baited benthic video landers (BVLs) in a Before-After Control-Impact (BACI) experimental design to test the acute effects of sediment deposition on the Dungeness crab (Cancer magister) and dog whelk (Nassarius spp). We considered the acute effects of both sediment deposition depths and the lateral surge (the turbidity front transiting the seafloor). Observations revealed sedimentation levels were limited (< 4 cm) and likely posed no direct threat to epifauna. Video and instrument readings showed the lateral surge to impact the BVLs as a 2 to 3 m/s sediment-laden front. Crabs were significantly impacted, while gastropods were more resistant to dislodgment. However, the high velocity impact was relatively brief (5 to 7 min). Further, crabs often returned to forage at BVLs after a mean lag of about 20 min post-impact. These results indicate an acute but ephemeral impact effect on crab, and support use of the thin-layer deposition method to minimize burial. The BVLs in a BACI experimental design were an effective means of measuring sediment impacts to mobile epifauna, and video observations were informative for understanding lateral surge dynamics and the behavioral interactions of organisms.

Structural variation in the brown alga Sargassum cymosum and its effects on associated amphipod assemblages

The presence of diverse biological substrates adds complexity to coastal landscapes and increases the number of ecological niches that can be used by the mobile epifauna. Studies on the influence of structural complexity have focused mainly on algal host species, but there is little information about the influence of intraspecific structural variation on the associated mobile epifauna. In this work, we examined whether intraspecific variation in the brown alga Sargassum cymosum influenced the structure of amphipod assemblages on two shores with different wave exposure. At least 15 fronds were randomly sampled at Fortaleza and Perequê-Mirim beaches, on the Atlantic coast of São Paulo state, southeastern Brazil, and 12 variables were measured for each alga. The amphipods were identified and counted. The greatest structural variation in S. cymosum occurred within shores, whereas the differences between shores were mainly related to algal size. These characteristics influenced amphipod assemblages differently on each shore, with the greatest effects being associated with variables related to morphological complexity, such as holdfast size, the number and size of branches, and the extent of cover by sessile colonial animals. These findings show that monospecific algal banks are not homogeneous, and that morphological differences and interactions with other biological substrates can influence the mobile epifaunal assemblages.

Distance effects on short-term recolonization of Sargassum stenophyllum by mobile epifauna, with an analysis of gammarid life habits

We evaluated experimentally whether distance between patches of the brown alga Sargassum stenophyllum relative to the source of colonizers influenced colonization patterns by (1) mobile epifauna and (2) gammarid amphipods with distinct life habits. Amphipods were the dominant group, commonly dispersing to patches up to 8 m distant from the algal bed, with low densities of other faunal groups. Assemblage structure of both faunal groups and gammarid amphipods on defaunated algae generally converged to that of controls during the experiments in less than eight days. Gammarid amphipods were dominated by tube-dwellers, but free-living nestlers also colonized the available substrate, and both reached control densities during the study. The same pattern was observed for representative species within each group. Colonization rates were related to faunal densities on the algal bed for most groups, suggesting that mobile organisms disperse more to new habitat patches in high density conditions. Thus, species with direct development can rapidly establish new aggregations at favourable sites, and contribute to small-scale heterogeneity in faunal distribution.