Pollution gradient leads to local adaptation and small-scale spatial variability of communities and functions in an urban marine environment

Urbanization of coastal habitats, of which harbors and marinas are the paragon, has led to various ecological paradigms about their functioning. Harbor infrastructures offer new hard substrata that are colonized by a wide variety of organisms (biofouling) including many introduced species. These structures also modify hydrodynamism and contaminant dispersal, leading to strong disturbance gradients within them. Differences in sessile community structure have previously been correlated to these gradients at small spatial scale (<100m). Local adaptation might be involved to explain such results, but as correlation is not causation, the present study aims to understand the causal link between the environmental gradients and community structure through a reciprocal transplant experiment among three sites of a marina (inner, middle, entrance). Our results highlighted strong small-scale spatial variations of contaminants (trace metals, PCB, pesticides, and PAH) in sediments and animal samples which have been causally linked to changes in community composition after transplant. But historical contingency and colonization succession also play an important role. Our results provided strong evidence for local adaptation since community structure, respiration, and pollutant uptake in Bugula neritina, as well as the metabolomes of B. neritina and Ciona intestinalis were impacted by the transplant with a disadvantage for individuals transplanted fromthe entrance to the inner location. The here observed results may thus indicate that the disturbance gradient in marinas might constitute a staple for selecting pollutantresistant species and populations, causing local adaptation. This highlights the importance of conducting further studies into small scale local adaptation.

Automated summary

The urbanization of coastal habitats, particularly harbors and marinas, has resulted in various ecological paradigms regarding their functioning. This study aimed to understand the causal link between environmental gradients and community structure in a marina through a reciprocal transplant experiment. The results provided strong evidence for local adaptation and highlighted the importance of disturbance gradients in selecting pollutant-resistant species and populations. Further studies on small-scale local adaptation are needed.