The ecology of natural and artificial hard substrata in marine coastal environments: substrate characteristics as facilitator of settlement and community stability

Abstract

Natural hard bottom ecosystems worldwide provide unique ecological functions and services within their respective environments. They protect coastlines by dampening large waves, reducing flooding, and preventing erosion. They act as nursery grounds for the associated fauna and play an important role in benthic-pelagic coupling, nutrient cycling and water purification. The rapid growth of the coastal infrastructure due to an ever-increasing population in coastal regions, as well as the growing challenge of global climate change (sea level rise, ocean acidification), are a serious threat to these ecosystems. The management focus on supporting local biodiversity in order to maintain natural ecosystem functioning and services is essential to safeguard a sustainable coastal development. Hence, while there is a strong need in protecting the still remaining natural hard bottom ecosystems, there is also an upcoming demand for reconsidering the way new coastal infrastructure is to be built. Engineering solutions need to focus on the objective to maintain local species richness, while also reducing the demand for natural resources in the production process of new building materials. The main objective of this doctoral project was to define drivers, which influence benthic community establishment in marine natural and artificial hard bottom systems. The influence of different physical environmental conditions and substrate characteristics in the settlement processes and community establishment was evaluated. Risks of invasion by neobiota in natural and artificial structures and options to implement natural ecosystem services in artificial environments as protection against invasion were discussed. The first part of this dissertation focusses on habitat characteristics of natural hard bottom systems of the southern North Sea, Germany. The southern North Sea is a marine environment with relatively low proportion of natural hard bottom ecosystems. Observation and monitoring of these protected grounds is often difficult due to the heterogeneous habitats and overall low visibility. Chapter 2 shows how video sampling methods in combination with environmental distribution maps on sediments, currents, and depth can help to model the status quo of biotic and abiotic conditions within a German nature conservation area, the “Helgoländer Steingrund” (HSG; 54°14.00N and 8°03.00W). Within this study, a new approach using species distribution models was tested on presence/absence data of nine benthic species (Echinus esculentus, Metridium senile, Cancer pagurus, Phymatolithon spp., Axinella polypoides, Homarus gammarus, Flustra foliacea, Alcyonidium diaphanum, Alcyonium digitatum). The species distribution models revealed good evaluation measures (true skill statistic >0.7; area under the receiver operation characteristic curve >0.90), implying that the model shows a good predictive performance. The outcome of this study is a clear recommendation on SDM application in further environmental monitoring programs on the HSG and other protected hard ground areas. Chapter 3 compares different hydroacoustic and hydrodynamic measurement tools to improve the assessments of the environmental conditions in the study area. Sonar systems, underwater videos, and bottom samples were used for mapping and classifying the abiotic and biotic components of the habitat. Based on acoustic backscatter data three main seabed types (sand, gravel, and hard substrate) were identified. The additional information from underwater videos and sediment samples lead to an expansion to six seabed types with different abiotic and biotic components. The flanks of the ridge of the HSG and their transition to the surrounding soft-ground areas were characterized by a distinct dominance of the bryozoa Flustra foliacea and Alcyonidium diaphanum. Acoustic Doppler Current Profiler data showed a uniform flow pattern across the ridge, and even resolved the local variability of current patterns, dependent of the tidal stage and bottom relief. Flow patterns are likely responsible for the zonation of the two benthic species. The second part of this dissertation focusses on habitat characteristics and succession on artificial substrates and environments. Those were in the focus of one-year succession studies presented in chapter 4 and chapter 5. The experiments described the establishment of benthic communities on concrete cubes (15 x 15 x 15 cm) made from five different concrete mixtures. The concrete mixtures contained various cements (Portland cement and blast furnace cements) and aggregates (sand, gravel, iron ore and blast furnace slag). Depending on their mixture, natural resources were saved, and CO2-emissions were reduced. This makes the materials “environmentally friendly”. All cubes were deployed in April 2017. After 12 months, they were examined regarding species composition and coverage, followed by statistical analysis (PERMANOVA, SIMPER, DIVERSE). One succession study was carried out in a natural hard bottom ecosystem near the island of Helgoland (chapter 4). The second study was conducted at the JadeWeserPort, Wilhelmshaven, as an example of a recently erected artificial habitat with high anthropogenic impact (chapter 5). Chapter 4 showed differences in settlement communities for different surface orientation of the cubes. Significant differences in settlement communities of the Front/Back side were present depending on the used concrete mixtures. Chapter 5 indicated marked differences in settled communities at the Port site compared to the natural environments of Helgoland. At the Port site community composition did not differ between the concrete mixtures. However, surface orientation of the cubes again revealed significant differences in species abundances and compositions. Cubes hold more neobiota in the Port site than in natural hard ground environments. Recommendations for the usage of “environmentally friendly” produced concrete mixtures are given. Regarding new coastal constructions, a sustainable production process of the required building materials should always be considered. As long as no significant difference in succession patterns and establishment of benthic communities between the “environmentally friendly” produced concrete mixtures and those that are commonly provided is present, “environmentally friendly” produced mixtures should be used. Anyhow, this is not sufficient to help maintaining ecosystem services under future scenarios of climate change. On the one hand, a protection of natural hard bottom systems is essential to maintain natural ecosystem service and functioning. On the other hand, the potential of artificial structures in restoring ecosystem services should not be underestimated. Artificial structures, if they fulfill certain criteria, as the use of environmentally friendly produced materials and combine those for instance with an enhanced habitat complexity, can, to certain extent compensate for natural habitat losses. If they host high biodiversity of native species, they can also be used to protect natural coasts from invasion by neobiota.