The findings within this study contribute to a better understanding of the process of settlement of benthic communities on artificial constructions, and further highlight that these structures cannot act as surrogates to natural grounds. In the context of a continuously increasing activity of constructing new marine coastal infrastructure [32,33,34], the investigation of new concrete mixtures is necessary with regards to resource efficiency, sustainability and economical aspects. There is more than one concrete type which can be used in coastal constructions, to safeguard natural resources. Testing different mixtures in the natural environment, is not only important for the evaluation of ecological impacts, but also concerning durability and static requirements.
In the JadeWeserPort, concrete mixture type made no differences to settled communities after one year. However, surface orientation of the cubes (Front/Top/Back) revealed significant differences in species abundances and community compositions. In a similar experimental setting, however in a less anthropogencially shaped environment, Becker et al. [44] observed different results. Here, the same concrete mixtures were exposed within the same period of time to the natural subtidal hard ground conditions of Helgoland Island. Becker et al. [44] also observed significant differences of settlement communities depending on the surface orientation of the cubes, but also significant differences in settled communities between mixture types. They suggests that concrete mixture type is negligible in anthropogenically influenced sites but more study sites are needed to confirm this. Impacts of artificial material in inshore coastal hard bottom communities might not necessarily be the same as for Helgoland, which, due to its relatively isolated location in the German Bight is offshore character [56].
Constructions of marine artificial infrastructure have been influencing natural environmental conditions for decades, for instance by changes in water flow, contamination loads, noise etc., and have impacted species richness and diversity which cannot be readily reversed [1, 57]. In nearshore environments, fragmentation of rocky shore habitats by replacing natural rock with artificial substrata, leads to a loss of habitat and changes the characteristics of the remaining assemblages [58]. The subdivision into numerous smaller habitat patches results in an overall reduction in species richness [59]. Species present in anthropogenically influenced sites are characterized by a generally broad range of tolerance [1]. Changes, for instance in concrete ingredients, will probably be of minor importance to those species. Natural hard grounds, in contrast, hold higher numbers of propagules, species that are more specialized and react more sensitive to small range environmental changes [1, 13, 57]. This might result in more drastic and visible changes in the settling community structures depending on mixture types.
Species composition and abundances in anthropogenically influenced and natural sites
Artificial constructions like ports, are known to differ from natural hard grounds in terms of community composition and species densities [57, 60]. Studies on artificial constructions found reduced species richness compared to the neighbouring natural communities [1, 61,62,63].
For the cubes deployed in the JadeWeserPort, a total of 32 taxa was found. This is low, compared to a total of 51 taxa found on the cubes of the natural hard ground study site in Helgoland by Becker et al. [44]. Comparing results of taxa numbers given by other studies conducted in the JadeWeserPort and Helgoland, the trend towards lower species diversity in the port site is also observed. For the JadeWeserPort a total of 116 taxa [48] is reported where at the natural site Helgoland, up to 402 taxa can be found [64]. Other studies show similar results. For instance, a recent comparison of concrete jetties versus natural rocky shores of the Mediterranean revealed a total of 150 algal and faunal taxa, 77 were recorded on jetties while 140 were recorded on natural rocky shores [65].
The floating pontoons in the JadeWeserPort were reported as the habitat of the harbor richest in species [48]. With a total of 63 taxa, they held more than half of all taxa found in the port site. Studies on floating and fixed artificial structures suggest, that the motion of floating structures, like pontoons, influence species composition and abundances [66, 67]. In temperate regions, differences between floating and fixed structures were mainly due to increased abundances of species [25, 66,67,68,69]. For tropical environments, changes in community composition were observed as well, for instance, more filter feeding organisms were found on floating structures, compared to fixed habitats [66, 67]. This might be explained by higher water flow and turbulence through these structures. This trend is also reflected by community composition found for the pontoons in the JadeWeserPort by Rhode et al. [48].
Regarding species composition, only eight red and brown algal species were found in the JadeWeserPort. The red alga Polysiphonia nigrescens dominated over all others covering most of the front sites of the cubes after the second month of deployment. Polysiphonia nigrescens was missing on the back side of the cubes, and here, barnacles dominated the surface. The back side of the cubes is shaded by the pontoons and since algae species need light for growth, it is reasonable that they preferred the open water side. However, taxa numbers of algae found on the concrete cubes in the port are considerably lower, compared to the natural study site, where cubes were covered by 23 algal taxa after one year of deployment. The proximity to natural rocky coastlines or reefs influence community characteristics on artificial constructions [57, 60]. The JadeWeserPort is surrounded by a mud flat environment. Thus, the pool of reproductive spores potentially reaching artificial structures to settle is low compared to the natural hard ground environments [13].
Water transparency in anthropogenic port sites is often considerably low, especially when a mud flat environment surrounds them. This is critical, as light availability is a main factor limiting the growth of algal species. Regular dredging activities in the port and in the channels close by, in combination with the regular tide flow, can increase the percentage of small mud particles in the water column [70,71,72,73]. In the JadeWeserPort, water transparency is already low in 0.5 to 1 m [48], as measured by Secchi depth. For the North Sea areas, water transparency increases with distance to the shore lines [74]. For the natural site Helgoland, mean water transparency lies already around 4–5 m Secchi depth [75, 76], a value which can hardly be measured in the proximity of anthropogenic construction sites.
Apart from a higher diversity of algae, more Bryozoan species were found at the natural study site as well, compared to the JadeWeserPort [44]. In addition to water transparency, water contamination levels influence species diversity. The southern parts of the North Sea are under the influence of the rivers Elbe, Weser, Ems, Rhein, Schelde and Thames. Hence, contamination levels with respect to brackish water inflow and industrial loads through these rivers are higher compared to the isolated position of Helgoland Island [56]. Studies from the Red Sea coast show that bryozoan species react sensitively to environmental pollution, mainly heavy metal contamination in soils. Diversity of Bryozoan species was higher in unpolluted areas than in anthropogenically influenced coastal sites [77]. Although we did not assess heavy metal load at the study site, it is likely that a higher contamination level in port sites may influence species diversity.
For the JadeWeserPort, the barnacle Balanus crenatus were among the characteristic species, especially for the cubes’ back sides. Barnacles are typical settling organisms on artificial structures worldwide [20,21,22,23, 78]. Within artificial constructions, barnacles prefer vertically orientated structures, but this can vary depending on sediment loads [79]. Other species typical for artificial constructions can be tube building polychaetes, like Spriobranchus triqueter or Spirobis spirobis for temperate regions, both missing from the JadeWeserPort study site [65, 79].
Neobiota
The experiments in the JadeWeserPort affirm a high invasion risk of artificial structures, as argued, for instance, in Glasby et al. [80]. All five neobiota found in this study were also included in the total of 14 neobiota found on the pontoon site in the JadeWeserPort by Rhode et al. [48]. However, abundances of neobiota were still low, compared to dominating native species. A fast succession of native competitors, for instance Polysiphonia nigrescens might have prevent the settlement by neobiota.
Since 1954, the barnacle Austominius modestus has been one of the main fouling species in German coastal waters [51]. After a series of mild winters and warm summers, exponential population growth was observed in several North Sea regions [51, 81]. This might become problematic with further increasing temperature due to climate change. A model on two competing barnacle species with different reproduction times (as would the case for Austrominius modestus and Balanus crenatus) revealed a positive impact of warming waters for invasive species due to a reduced time period between the reproductive peaks of the species [82]. However, native species can be supported and positively influenced by the precise timing of the introduction of new substrates [82]. In the present study, the native barnacle Balanus crenatus still dominated on the cubes of the JadeWeserPort. The slipper snail Crepidula fornicata, also introduced from England where it spread to most European ports, has been established as part of the German marine fauna since 1934. Its abundance is still strongly reduced by cold winters [51]. On the cubes in the JadeWeserPort, the pacific oyster Crassostera gigas was also found being introduced to the North Sea waters in the middle of the twentieth century where in the Wadden Sea, it replaced native mussel beds of Mytilus edulis. Hitherto, Crassostera gigas can be found along almost all European coastlines [51]. At the natural study site of Helgoland Becker et al. [44] observed two neobiota (Botryllus schlosseri and Bonnemaisonia hamifera) but they do not seem to have a negative impact on the natural environment [51].
There are several reasons given as to why artificial structures are particularly vulnerable to invasion. Those reasons entail a generally lower diversity of native species, reduced competitive interaction and predation risk, but also changes in environmental conditions, like a reduced water flow in more sheltered conditions [1]. For breakwaters along the coasts of Italy a spread of introduced green macroalgae has been found [12, 24]. Algae benefit from the wave-sheltered environments on the shoreward side of the breakwaters [12, 24]. Dafforn et al. [28] argue that filter-feeding invaders, which are often transported on ship hulls, could take advantage of being adapted to high shear stress by colonizing open space on moving substrata, for instance floating docks. Regarding reduced competitive interactions and predation risk, as postulated by the biotic resistance theory [29] and enemy release hypotheses [30], it also needs to be taken into account that artificial structures always initiate primary succession when they are built or released to marine environments. It is difficult to predict, if neobiota will manage to replace native competitors in the long term. However, with ongoing climate change and global trade and transport it is likely that neobiota will succeed over native species [83, 84].
In conclusion, a general recommendation can be given with respect to the use of new concrete mixtures in marine constructions. As long as there is no significant difference in succession patterns and establishment of benthic communities between the new concrete mixtures and those which are commonly provided, and that leakage of environmental pollutants can be excluded, the new mixtures should be used for new constructions. This way, at least a more environmentally friendly production would be guaranteed. However, it is important to balance between costs and benefits of new concrete mixtures and building solutions may differ from case to case.