Anthropogenic disturbance can determine the magnitude of opportunistic species responses on marine urban infrastructures

Abstract

Background: Coastal landscapes are being transformed as a consequence of the increasing demand for infrastructures to sustain residential, commercial and tourist activities. Thus, intertidal and shallow marine habitats are largely being replaced by a variety of artificial substrata (e.g. breakwaters, seawalls, jetties). Understanding the ecological functioning of these artificial habitats is key to planning their design and management, in order to minimise their impacts and to improve their potential to contribute to marine biodiversity and ecosystem functioning. Nonetheless, little effort has been made to assess the role of human disturbances in shaping the structure of assemblages on marine artificial infrastructures. We tested the hypothesis that some negative impacts associated with the expansion of opportunistic and invasive species on urban infrastructures can be related to the severe human disturbances that are typical of these environments, such as those from maintenance and renovation works.

Methodology/principal findings: Maintenance caused a marked decrease in the cover of dominant space occupiers, such as mussels and oysters, and a significant enhancement of opportunistic and invasive forms, such as biofilm and macroalgae. These effects were particularly pronounced on sheltered substrata compared to exposed substrata. Experimental application of the disturbance in winter reduced the magnitude of the impacts compared to application in spring or summer. We use these results to identify possible management strategies to inform the improvement of the ecological value of artificial marine infrastructures.

Conclusions/significance: We demonstrate that some of the impacts of globally expanding marine urban infrastructures, such as those related to the spread of opportunistic, and invasive species could be mitigated through ecologically-driven planning and management of long-term maintenance of these structures. Impact mitigation is a possible outcome of policies that consider the ecological features of built infrastructures and the fundamental value of controlling biodiversity in marine urban systems.

Figure 1. Human-made, coastal defence infrastructures at Cesenatico, along the Italian shores of the north Adriatic sea.

A) View of the study area and breakwaters from Google Earth. B) View of a breakwater under maintenance (photo L. Airoldi).

Figure 2. Two-dimensional plots of principal coordinate axes (unconstrained metric multi-dimensional scaling, MDS).

The MDS shows ordination of centroids of assemblages at the landward and seaward sides of replicated control (non-maintained) and maintained (March 2002) breakwaters at Cesenatico in May 2002, August 2002, January 2003 and May 2003. There were 4 control and 4 maintained breakwaters, except for May 2003, when there were only 2 control breakwaters. Analyses were based on Bray-Curtis dissimilarities after 4th-root transformation of cover data. Stress values lower than 0.10 indicate that the ordination is good and that the interpretation of patterns in 2 dimensions is reliable.

Figure 3. Abundance of most abundant taxa on the study breakwaters at Cesenatico.

Data are average percent covers ±1 SE (n = 32, 8 replicate plots for each of 4 breakwaters; in May 2003 n = 16 for controls, 8 replicate plots for each of 2 breakwaters) of Mytilus galloprovincialis, oysters (a mixture of Ostrea edulis and Crassostrea gigas difficult to separate by visual sampling), macroalgae (mainly Ulva spp., Codium fragile ssp. tomentosoides and filamentous forms), biofilm (a coating of microalgae, juvenile stages of macroalgae and silt) and bare rock (rock non occupied by visible macroscopic forms) at the landward and seaward sides of control (non-maintained) and maintained (April 2002) breakwaters, in May 2002, August 2002, January 2003 and May 2003. Asterisks indicate significant differences between assemblages on maintained and control breakwaters as indicated by a posteriori SNK tests.

Figure 4. Two-dimensional plots of principal coordinate axes (metric multi-dimensional scaling, MDS).

The MDS shows ordination of centroids of assemblages at the landward and seaward sides of each of 3 breakwaters for each time of disturbance (D1 =  April 2003, D2 =  August 2003, D3 =  January 2004, C =  unmanipulated plots). Analyses were based on Bray-Curtis dissimilarities after 4th-root transformation of data collected in May 2004. A Stress value of 0.05 indicates that the ordination is excellent and that the interpretation of patterns in 2 dimensions is highly reliable.

Figure 5. Abundance of macroalgae in the experimental treatments at Cesenatico.

Data are average percent covers ±1 SE (n = 36, 4 replicate plots for each of 4 blocks for each of 3 breakwaters) of macroalgae (mainly Ulva spp. and Codium fragile ssp. tomentosoides) at the landward and seaward sides of breakwaters for each time of disturbance (April 2003, August 2003, January 2004, Control  =  unmanipulated plots).