Natural Hard Substrates Research Articles

Who lives in a pear tree under the sea? A first look at tree reefs as a complex natural biodegradable structure to enhance biodiversity in marine systems

Hard substrates play an important role in global marine systems as settlement surface for sessile reef-forming species such as corals, seaweeds, and shellfish. In soft-sediment systems, natural hard substrates such as stones, bedrock and driftwood are essential as they support diverse assemblages of reef-associated species. However, availability of these hard substrates has been declining in many estuaries and shallow seas worldwide due to human impacts. This is also the case in the Dutch Wadden Sea, where natural hard substrates have gradually disappeared due to burial by sand and/or active removal by humans. In addition, driftwood that was historically imported from rivers has been nullified by upstream logging and coastal damming of estuaries. To investigate the historic ecological role of wood presence in the Wadden Sea as settlement substrate and fish habitat, we constructed three meter high artificial reefs made of felled pear trees. Results demonstrate that these reefs rapidly developed into hotspots of biodiversity. Within six months, the tree-reefs were colonized by sessile hard substrate associated species, with a clear vertical zonation of the settled species. Macroalgae and barnacles were more abundant on the lower parts of the reef, while bryozoans were more dominant on the upper branches. In addition, six fish species were observed on the reefs, while only two species were caught on sandy control sites. Moreover, the abundance of fish on the reefs was five times higher. Individuals of the most commonly caught species, the five-bearded rockling Ciliata mustela, were also larger on the reef. These patterns also hold true for common prawn, Palaemon serratus, which were also larger and ten times more numerous on the reefs. Present findings indicate that the reintroduction of tree-reefs as biodegradable, structurally complex hard substrates can increase local marine biodiversity in soft-sediment systems within relatively short time scales.

Vertical distribution of macrobenthic communities from a shipwreck in the Strait of Magellan

Shipwrecks are artificial reefs that can enhance local diversity. In this study, the macrobenthic community associated with a shipwreck in the Strait of Magellan, the DAP Mares, is described for the first time. Species richness and functional group richness were estimated, and general linear models were used to test the prediction that diversity varied systematically with depth. The most speciose and abundant groups were echinoderms, molluscs, sponges, and tunicates. The most abundant trophic type was suspension feeders. Species richness changed significantly with depth, being similar between the upper and middle zones but decreasing at the deepest zone. Neither functional group richness nor functional dispersion (Rao’s quadratic entropy) varied with depth, suggesting that not all aspects of macrobenthic diversity respond equally to this variable. The biodiversity observed was consistent with that described for natural hard substrates in the Strait of Magellan. Thus, this vessel could represent a good model to study patterns of community structure.

Offshore platforms as novel ecosystems: A case study from Australia's Northwest Shelf.

The decommissioning of offshore oil and gas platforms typically involves removing some or all of the associated infrastructure and the consequent destruction of the associated marine ecosystem that has developed over decades. There is increasing evidence of the important ecological role played by offshore platforms. Concepts such as novel ecosystems allow stakeholders to consider the ecological role played by each platform in the decommissioning process. This study focused on the Wandoo field in Northwest Australia as a case study for the application of the novel ecosystem concept to the decommissioning of offshore platforms. Stereo‐baited remote underwater video systems were used to assess the habitat composition and fish communities at Wandoo, as well as two control sites: a sandy one that resembled the Wandoo site pre‐installation, and one characterized by a natural reef as a control for natural hard substrate and vertical relief. We found denser macrobenthos habitat at the Wandoo site than at either of the control sites, which we attributed to the exclusion of seabed trawling around the Wandoo infrastructure. We also found that the demersal and pelagic taxonomic assemblages at Wandoo more closely resemble those at a natural reef than those which would likely have been present pre‐installation, but these assemblages are still unique in a regional context. The demersal assemblage is characterized by reef‐associated species with higher diversity than those at the sand control and natural reef control sites, with the pelagic community characterized by species associated with oil platforms in other regions. These findings suggest that a novel ecosystem has emerged in the Wandoo field. It is likely that many of the novel qualities of this ecosystem would be lost under decommissioning scenarios that involve partial or complete removal. This study provides an example for classifying offshore platforms as novel ecosystems.

Material type influences the abundance but not richness of colonising organisms on marine structures

Urbanisation of coastal areas and growth in the blue economy drive the proliferation of artificial structures in marine environments. These structures support distinct ecological communities compared to natural hard substrates, potentially reflecting differences in the materials from which they are constructed. We undertook a meta-analysis of 46 studies to compare the effects of different material types (natural or eco-friendly vs. artificial) on the colonising biota on built structures. Neither the abundance nor richness of colonists displayed consistent patterns of difference between artificial and natural substrates or between eco-friendly and standard concrete. Instead, there were differences in the abundance of organisms (but not richness) between artificial and natural materials, that varied according to material type and by functional group. When compared to biogenic materials and rock, polymer and metal supported significantly lower abundances of total benthic species (in studies assessing sessile and mobile species together), sessile invertebrates and corals (in studies assessing these groups individually). In contrast, non-indigenous species were significantly more abundant on wood than metal. Concrete supported greater abundances of the general community, including habitat-forming species, compared to wood. Our results suggest that the ecological requirements of the biological community, alongside economic, logistic and engineering factors should be considered in material selection for multifunctional marine structures that deliver both engineering and ecological (enhanced abundance and diversity) benefits.

Experimental apparatus for investigating colonization, succession and related processes of rocky bottom epifauna

To fully understand what controls the structure of benthic assemblages, recognition of the factors that influence each step of their development is needed. This includes knowledge about crucial processes such as biota recruitment and succession. Complex, heterogeneous natural substrates occurring on the sea floor, such as rocks, boulders and biogenic substrata, are often difficult to replicate experimentally. Uniform artificial substrate in the form of plates, often referred to as settlement panels, mimic the natural hard substrata of the rocky bottom and provides uniform replication at a wider scale. This allows scientists to study recruitment and succession processes and also various other issues related to colonization in aquatic environments. In this paper, we present detailed instructions for the construction of a settlement panel apparatus that has been used successfully over multiple years to study epifaunal succession and colonization in shallow-water marine environments around the world; we also provide practical examples of such studies from temperate and polar areas.

Subtidal Natural Hard Substrate Quantitative Habitat Mapping: Interlinking Underwater Acoustics and Optical Imagery with Machine Learning

Subtidal natural hard substrates (SNHS) promote occupancy by rich benthic communities that provide irreplaceable and fundamental ecosystem functions, representing a global priority target for nature conservation and recognised in most European environmental legislation. However, scientifically validated methodologies for their quantitative spatial demarcation, including information on species occupancy and fine-scale environmental drivers (e.g., the effect of stone size on colonisation) are rare. This is, however, crucial information for sound ecological management. In this investigation, high-resolution (1 m) multibeam echosounder (MBES) depth and backscatter data and derivates, underwater imagery (UI) by video drop-frame, and grab sediment samples, all acquired within 32 km2 of seafloor in offshore Belgian waters, were integrated to produce a random forest (RF) spatial model, predicting the continuous distribution of the seafloor areal cover/m2 of the stones’ grain sizes promoting colonisation by sessile epilithic organisms. A semi-automated UI acquisition, processing, and analytical workflow was set up to quantitatively study the colonisation proportion of different grain sizes, identifying the colonisation potential to begin at stones with grain sizes Ø ≥ 2 cm. This parameter (i.e., % areal cover of stones Ø ≥ 2 cm/m2) was selected as the response variable for spatial predictive modelling. The model output is presented along with a protocol of error and uncertainty estimation. RF is confirmed as an accurate, versatile, and transferable mapping methodology, applicable to area-wide mapping of SNHS. UI is confirmed as an essential aid to acoustic seafloor classification, providing spatially representative numerical observations needed to carry out quantitative seafloor modelling of ecologically relevant parameters. This contribution sheds innovative insights into the ecologically relevant delineation of subtidal natural reef habitat, exploiting state-of-the-art underwater remote sensing and acoustic seafloor classification approaches.

Benthic community establishment on different concrete mixtures introduced to a German deep-water port

Concrete is a widely used building material in coastal constructions worldwide. However, limited natural resources used in the production process, as well as high CO2-emission due to the calcination process of limestone and the thermal energy demand for Portland cement clinker production, raise the demand for alternative constituents. Alternative mixture types should be environmentally friendly and, at best, mimic natural hard substrates. Here five different concrete mixtures, containing different cements (Portland cement and blast furnace cements) and aggregates (sand, gravel, iron ore and metallurgical slags) were made. Three replicate cubes (15 × 15 × 15 cm) of each type were then deployed in a German deep-water Port, the JadeWeserPort, to study benthic community establishment after one year. Results are compared to a similar experiment conducted in a natural hard ground environment (Helgoland Island, Germany). Results indicate marked differences in settled communities in the Port site compared to natural environments. At the Port site community composition did not differ with the concrete mixtures. Surface orientation of the cubes (front/top/back) revealed significant differences in species abundances and compositions. Cubes hold more neobiota in the Port site than in natural hard ground environments. Implications for the usage of new concrete mixtures are discussed.

A comparison of marine communities along a subsea pipeline with those in surrounding seabed areas

The value of subsea pipelines as habitat for fish and benthic species is being considered, particularly by the oil and gas industry as they look to decommission seafloor infrastructure reaching the end of production. We investigated fish and benthic communities along a ca.345 km section of offshore pipeline in remote northern Australia to compare pipeline communities with those in surrounding areas and provide context for decommissioning risk assessments. We surveyed pipeline and adjacent natural seabed ecosystems at five locations on the continental shelf using remote video technology to quantify fish and benthic communities and modelled predicted fish communities across the study locations. We found that the pipeline supported turfing and low-relief biota (e.g. coralline algae, ascidians, bryozoans, small/encrusting sponges and soft coral or mixed filter feeder communities) with cover >75%. Pipeline benthic communities differed from those in adjacent ecosystems. Within 6 km of the pipeline, the seabed was predominantly sand/silt with only sparse biota; natural hard substrate (e.g. shoal features within 3 km of the pipeline) supported diverse coral and filter feeder communities (average 10–25% cover). Fish abundance and assemblage composition on the pipeline also differed from those in surrounding high- and low-complexity habitats. This difference was driven by high abundance and biomass of commercially important fish species near the pipeline and sand-affiliated species elsewhere. Our study provides important new insights into the marine communities associated with a subsea pipeline in northern Australia; with this artificial habitat observed to support a subset of species known to inhabit the area, but effectively representing a unique assemblage within the region.

Influence of concrete properties on the initial biological colonisation of marine artificial structures

Artificial marine infrastructures now cover large stretches of the available natural shoreline in many parts of the world. This is having a substantial impact on the local marine ecosystems as biodiverse natural hard substrata are being replaced with man-made structures, which have been shown to support lower levels of biodiversity.The ecological value of artificial coastal structures could be enhanced through careful design of pre-fabricated ecologically engineered units. Material selection is a critical parameter in the design of these units. To maximise the potential of concrete units to support and increase biodiversity, this paper explores the impact of binder composition, aggregate type and plasticizer on surface chemistry and early biofilm formation which influence subsequent colonisation. Experiments in the current study have shown that the addition of ground granulated blast-furnace slag (GGBS) to the mix can increase the levels of biodiversity supported while maintaining engineering performance requirements in terms of strength, chloride resistance and alkalinity. Conversely the aggregate type and use of a plasticizer have a minor influence. In particular, the addition of GGBS was shown to increase the biomass of diatoms, cyanobacteria and green algae and barnacle abundance one month after immersion on both sheltered surfaces and those exposed to wave action. Results suggest that concrete composition can alter the surface chemistry of artificial structures and thereby can improve the ecological value of these structures as habitats for marine life.

Succession in epibenthic communities on artificial reefs associated with marine renewable energy facilities within a tide-swept environment

AbstractAlthough colonization of artificial structures by epibenthic communities is well-documented overall, our understanding of colonization processes is largely limited to low-energy environments. In this study, we monitored epibenthic colonization of different structures associated with a tidal energy test site located in a high-energy hydrodynamic environment. Using four years of image-based underwater surveys, we characterized changes through space and time in the taxonomic composition of epibenthic assemblages colonizing two kinds of artificial structures, as well as the surrounding natural habitat. Our results highlight that ecological successions followed similar trends across the two artificial habitats, but that different habitat-specific communities emerged at the end of our survey. Deployment of these artificial structures resulted in the addition of elevated and stable substrata in an environment where natural hard substrates are unstable and strongly exposed to sediment abrasion. Although epibenthic communities colonizing artificial habitats are unlikely to have reached a mature stage at the end of our survey, these supported structurally complex taxa facilitating an overall increase in local diversity. We were able to quantify how epibenthic communities can significantly vary over time in high-energy coastal environment, and our final survey suggests that the ecological succession was still in progress five years after the deployment of artificial reefs. Thus, maintaining long-term continuous survey of coastal artificial reef habitats will be key to better discriminate between long-term ecological successions and shorter-term variability.

Drought periods driving bioinvasion on hard substrates at a tropical estuary, Eastern Brazil

During drier periods estuarine salinization can stimulate the colonization of marine organisms and further bioinvasion, therefore, may cause future ecological change. In this sense, we applied the Rapid Assessment Surveys (RAS) on natural and artificial hard substrates during the most intense drought period (between 2014 and 2016) since 1950, in a tropical estuary in Eastern Brazil. Through the estuary salinization the ascidian Didemnum psammatodes overgrow the dominant native taxa; this dominance taxa shift provided suitable habitat favorable for invasion of Ophiothela mirabilis across the estuary. Even the invasion starts on artificial substrates, the higher resilience of natural substrates were not enough to enable the establishment of invasive species. This baseline is the first evidence of bioinvasion influenced by climate change drought periods in tropical estuaries in South America, which is a great advance for the development of management strategies to mitigate this and future climate change scenarios.

Understanding the effects of coastal defence structures on marine biota: The role of substrate composition and roughness in structuring sessile, macro- and meiofaunal communities

The increasing deployment of artificial structures into the marine environment is creating new hard substrates that differ from natural ones in physical and biological aspects. However, studies of macrofaunal and meiofaunal communities associated with artificial structures are very limited. Seawalls, cubes, acropods and rip-raps in Algeciras Bay (southern Spain) were each compared with the nearest natural hard substrate and their community structure was related to substrate roughness, composition, carbonates content, crystallinity and age, using db-RDA. The results showed clear differences between substrates for the three community levels (sessile, macro- and meiofauna). Overall, rip-raps were the most similar to natural substrates. Under similar environmental conditions, substrate roughness, composition (only for sessile) and age of the structures seemed to play important roles in structuring those communities. They especially affected the sessile community, initiating strong cascading effects that were detectable at high taxonomic level in the associated fauna.

From sessile to vagile: Understanding the importance of epifauna to assess the environmental impacts of coastal defence structures

Ocean sprawl is leading to the introduction of multiple artificial structures into the marine environment. However, the biota on these novel habitats differ from that on natural hard substrates. Amphipods, despite their ecological importance, are usually overlooked when comparing benthic assemblages on artificial and natural hard substrates. So as to assess the effects of artificial structures on amphipod assemblage and to identify the main factors involved, the amphipod assemblage structure was studied in five different substrates (seawalls, cubes, acropods, rip-raps and natural rock). Abiotic measurements of each substrate (complexity, rock composition, and age) were related to the ecological patterns. Complexity measurements seemed to affect the amphipod community structure, highlighting the need to consider physical complexity in eco-engineering actions. Amphipod assemblages were also affected by the secondary substrate (sessile biota), suggesting that artificial structures are indirectly shaping amphipod assemblages by firstly shaping the sessile biota. Future research should study the same secondary substrates across different artificial structures to separate the direct effects (caused by the artificial structures) from the indirect effects (caused by the sessile biota).

Social network analysis as a tool for marine spatial planning: Impacts of decommissioning on connectivity in the North Sea

Abstract Connectivity of marine populations and ecosystems is crucial to maintaining and enhancing their structure, distribution, persistence, resilience and productivity. Artificial hard substrate, such as that associated with oil and gas platforms, provides settlement opportunities for species adapted to hard substrates in areas of soft sediment. The contribution of artificial hard substrate and the consequences of its removal (e.g. through decommissioning) to marine connectivity is not clear, yet such information is vital to inform marine spatial planning and future policy decisions on the use and protection of marine resources. This study demonstrates the application of a social network analysis approach to quantify and describe the ecological connectivity, informed by particle tracking model outputs, of hard substrate marine communities in the North Sea. Through comparison of networks with and without artificial hard substrate, and based on hypothetical decommissioning scenarios, this study provides insight into the contribution of artificial hard substrate, and the consequence of decommissioning, to the structure and function of marine community connectivity. This study highlights that artificial hard substrate, despite providing only a small proportion of the total area of hard substrate, increases the geographic extent and connectivity of the hard substrate network, bridging gaps, thereby providing ‘stepping stones’ between otherwise disconnected areas of natural hard substrate. Compared to the baseline scenario, a decommissioning scenario with full removal of oil and gas platforms results in a nearly 60% reduction in connectivity. Such reduction in connectivity may have negative implications for species’ distribution, gene flow and resilience following disturbance or exploitation of marine hard substrate communities. Synthesis and applications. Social network analysis can provide valuable insight into connectivity between marine communities and enable the evaluation of impacts associated with changes to the marine environment. Providing standardized, transparent and robust outputs, such a tool is useful to facilitate understanding across different disciplines, including marine science, marine spatial planning and marine policy. Social network analysis therefore has great potential to address current knowledge gaps with respect to marine connectivity and crucially facilitate assessment of the impacts of changes in offshore substrate as part of the marine spatial planning process, thereby informing policy and marine management decisions.

Annelid assemblages on artificial and natural hard substrata differ decades after building: A case study on a one-century old seawall

Although strong differences have been observed between the assemblages on artificial substrata and on natural hard-bottom habitats, little is known about the mechanisms that cause them. The purpose of this study was to examine the differences in Mediterranean low shore annelid assemblages dwelling on an old artificial, vertical hard substratum and a natural, horizontal one. The structure of the Ellisolandia elongata turfs covering the two habitats showed little divergence in terms of length and number of thalli, as well as in available space for epibionts. In this respect, colonization duration might explain the lack of significant dissimilarities in the phytal canopy. In spite of this, statistical analyses showed that the specific composition of the two annelid assemblages differed in a significant manner. Although they shared a set of constant species, their relative abundances were dissimilar and this was especially evident for several species belonging to families Syllidae and Enchytraeidae. Abundance and species richness were significantly higher on natural structures, as expected, but Shannon diversity and equitability indexes were lower. Apparently, the sciophilous condition of vertical surfaces, which do not occur naturally in the area, is the main driver of the differences in species composition and its capital importance remains after colonization processes have occurred.

Comparing sessile benthos on shallow artificial versus natural hard substrates in the Eastern Mediterranean Sea

Artificial structures cover a considerable part of the Mediterranean coasts. In the Aegean Sea, most studies related to artificial structures have focused in vagile fauna on harbors and marinas but little attention has been given to the sessile biota on coastal defense structures. The aim of this work was to describe for the first time the shallow sublittoral sessile benthos on coastal defense structures in Crete (Eastern Mediterranean Sea) in order to identify potential differences in comparison to natural rocky substrates, adopting both a taxonomic and functional (i.e. macroalgal structural complexity) approach. Three shallow (1-3 m) localities were studied in the north coast and three in the south coast of the island (six localities in total). At each locality, two types of hard substrate were selected: an artificial coastal defense structure (rip-rap) and the nearest natural rocky substrates. The percent cover of sessile taxa was calculated using random points counts over photoquadrats (20 x 20 cm). The structure of the assemblage differed between artificial and natural habitats. Values of Shannon-Wiener’s diversity index and number of taxa were higher in natural substrates. In addition, cover of arborescent macroalgae was lower on artificial substrates. In conclusion, rip-raps do not function as surrogates of natural hard substrates in the study area since their shallow subtidal assemblages differ in terms of community structure, diversity and functionality. The deficient performance of such artificial structures could be attributed to the combined effects of abiotic factors and biotic processes, including substrate nature and roughness as well as differential grazing pressure.

Connectivity of larval stages of sedentary marine communities between hard substrates and offshore structures in the North Sea

Man-made structures including rigs, pipelines, cables, renewable energy devices, and ship wrecks, offer hard substrate in the largely soft-sediment environment of the North Sea. These structures become colonised by sedentary organisms and non-migratory reef fish, and form local ecosystems that attract larger predators including seals, birds, and fish. It is possible that these structures form a system of interconnected reef environments through the planktonic dispersal of the pelagic stages of organisms by ocean currents. Changes to the overall arrangement of hard substrate areas through removal or addition of individual man-made structures will affect the interconnectivity and could impact on the ecosystem. Here, we assessed the connectivity of sectors with oil and gas structures, wind farms, wrecks, and natural hard substrate, using a model that simulates the drift of planktonic stages of seven organisms with sedentary adult stages associated with hard substrate, applied to the period 2001–2010. Connectivity was assessed using a classification system designed to address the function of sectors in the network. Results showed a relatively stable overall spatial distribution of sector function but with distinct variations between species and years. The results are discussed in the context of decommissioning of oil and gas infrastructure in the North Sea.

Effects of different intertidal hard substrates on the recruitment of Crassostrea gigas

Physical properties, distribution, and abundance of natural hard substrates are important variables modulating the potential distribution of sessile aquatic species. In this study, we analyze the effects of the geological origin of different rock types common in intertidal environments of Argentina on the recruitment success of the nonindigenous Pacific oyster Crassostrea gigas, accounting for spatial and interannual variation. A series of experiments showed that although recruitment occurs on all of the five most widespread hard substrates in the region (i.e., pelitic mudstone, limestone, conglomeradic sabulitic sandstone, gray sandstone, and volcanic rock) it is significantly higher on pelitic mudstone. Accounting for the geographic distribution of these substrate types along the coast of Argentina, it is concluded that the Valdes Peninsula and central San Jorge Gulf areas, where pelitic mudstone outcrops are the dominant intertidal hard substrate, are the regions with highest potential for C. gigas intertidal reef formation. Pelitic mudstone displayed the highest mean values of specific heat and porosity as well as high values of luminosity.

CARACTERIZAÇÃO DA MACROFAUNA DE SUBSTRATOS CONSOLIDADOS DO LITORAL NORTE DO RIO GRANDE DO SUL, BRASIL

Este trabalho teve como objetivo realizar um levantamento qualitativo e descrever a zonação dos organismos incrustantes, sedentários e vágeis em diferentes substratos consolidados na costa norte do Rio Grande do Sul (RS), Brasil, contribuindo para o conhecimento da biodiversidade subtropical marinha. Os registros das espécies foram feitos através de consulta à literatura específica e também através de observações e coletas mensais em substratos consolidados naturais (costão rochoso) e artificiais (molhes, plataformas, monoboias) de março de 2010 a junho de 2011, totalizando 16 meses. Obteve-se o registo de 160 espécies de invertebrados associados a substratos consolidados para o RS através da literatura e 83 espécies a partir do levantamento de campo, sendo que 42,2 % das espécies foram registradas no costão rochoso da Praia do Meio, Torres; 24,1 % nos molhes de Imbé-Tramandaí e Torres; 45,8 % nos pilares das plataformas de pesca de Cidreira, Tramandaí e Atlântida e 72,3 % na estrutura das monoboias de oleoduto, MN601 e MN-602 de Tramandaí. Sobre a ocorrência exclusiva de espécies, verificaram-se maiores valores para as monoboias, seguida das plataformas de pesca, costão rochoso e molhes. Quanto a novos registros, 34 táxons ainda não haviam sido citados para RS.

Beta diversity of macroalgal communities around St. Eustatius, Dutch Caribbean

This study provides a baseline of the marine algal flora composition around St. Eustatius, Dutch Caribbean, by describing algal community structure in terms of species richness and beta diversity, and by providing a taxonomically reliable DNA barcode collection. A total of 156 species was found, including 91 that represent new records for St. Eustatius. Subtidal assemblages (126 species) and intertidal assemblages (48 species) showed little overlap. Algae assemblages in seagrass beds differed from those on hard substrates in species composition. In addition, seagrass communities contained a relatively high number of associated green algae species. Artificial substrates (such as shipwrecks) mimicked natural hard substrates in terms of species richness and composition, but missed some key species that characterize natural reef floras. Species accumulation curves and asymptotic species richness estimators show that the expected species richness is higher than the observed number of species, indicating that additional sampling is needed to record rare species. The phylogenetic trees provided in this study identified the presence of cryptic species and fills knowledge gaps in our understanding of Caribbean macroalgae.