Tim M. Glasby

Marine urbanization: an ecological framework for designing multifunctional artificial structures

Underwater cities have long been the subject of science fiction novels and movies, but the “urban sprawl” of artificial structures being developed in marine environments has widespread ecological consequences. The practice of combining ecological principles with the planning, design, and operation of marine artificial structures is gaining in popularity, and examples of successful engineering applications are accumulating. Here we use case studies to explore marine ecological engineering in practice, and introduce a conceptual framework for designing artificial structures with multiple functions. The rate of marine urbanization will almost certainly escalate as “aquatourism” drives the development of underwater accommodations. We show that current and future marine developments could be designed to reduce negative ecological impacts while promoting ecosystem services.

Shallow moving structures promote marine invader dominance.

Global increases in urban development have resulted in severe habitat modification in many estuaries. Most are now dominated by artificial structures, which might have a myriad of effects on native species. The provision of extra hard substrata presents additional free space, and recent research suggests non-indigenous epifauna may be able to exploit these artificial structures (particularly pontoons) more effectively than native species. The early development of fouling assemblages was compared on settlement plates attached to fixed or moving experimental structures at depths of 0.5 m and 2 m. Invertebrate invaders as a group were disproportionately more numerous on shallow, moving plates (essentially floating surfaces) than on deeper plates, whereas native epifauna were less numerous than invaders in all treatments. Importantly, however, individual invasive species showed differing effects of movement and depth. Future management strategies should take into account the potential for shallow, moving structures to enhance invader dominance and strongly consider using fixed structures to reduce opportunities for invaders.

Differential effects of tributyltin and copper antifoulants on recruitment of non-indigenous species

Maritime transport is a primary vector for many marine invaders. For the past two decades, most commercial vessels have used tributyltin (TBT) antifouling (AF) paint, whereas recreational vessels have been restricted to alternatives, most commonly containing copper. Settlement plates painted with a collar of copper or TBT AF paint, and unpainted control plates, were deployed in commercial and recreational embayments in Port Jackson, Australia, and sampled photographically after 5 and 10 months. Copper enhanced early recruitment of several non-indigenous species (NIS), whereas recruitment of indigenous species was typically reduced by copper. TBT limited the recruitment of NIS for just 5 months and indigenous species, for the entire study. The results suggest that the use of toxic AF paints, and the possible accumulation of AF biocides in embayments, may be negatively affecting indigenous epibiota. Conversely, copper antifoulants on recreational vessels may be facilitating the transport and establishment of copper tolerant NIS into disturbed estuarine habitats.

Comparing the Invasibility of Experimental “Reefs” with Field Observations of Natural Reefs and Artificial Structures

Natural systems are increasingly being modified by the addition of artificial habitats which may facilitate invasion. Where invaders are able to disperse from artificial habitats, their impact may spread to surrounding natural communities and therefore it is important to investigate potential factors that reduce or enhance invasibility. We surveyed the distribution of non-indigenous and native invertebrates and algae between artificial habitats and natural reefs in a marine subtidal system. We also deployed sandstone plates as experimental ‘reefs’ and manipulated the orientation, starting assemblage and degree of shading. Invertebrates (non-indigenous and native) appeared to be responding to similar environmental factors (e.g. orientation) and occupied most space on artificial structures and to a lesser extent reef walls. Non-indigenous invertebrates are less successful than native invertebrates on horizontal reefs despite functional similarities. Manipulative experiments revealed that even when non-indigenous invertebrates invade vertical “reefs”, they are unlikely to gain a foothold and never exceed covers of native invertebrates (regardless of space availability). Community ecology suggests that invertebrates will dominate reef walls and algae horizontal reefs due to functional differences, however our surveys revealed that native algae dominate both vertical and horizontal reefs in shallow estuarine systems. Few non-indigenous algae were sampled in the study, however where invasive algal species are present in a system, they may present a threat to reef communities. Our findings suggest that non-indigenous species are less successful at occupying space on reef compared to artificial structures, and manipulations of biotic and abiotic conditions (primarily orientation and to a lesser extent biotic resistance) on experimental “reefs” explained a large portion of this variation, however they could not fully explain the magnitude of differences.

Building ‘blue’: An eco-engineering framework for foreshore developments

Urbanisation in terrestrial systems has driven architects, planners, ecologists and engineers to collaborate on the design and creation of more sustainable structures. Examples include the development of green infrastructure and the introduction of wildlife corridors that mitigate urban stressors and provide positive ecological outcomes. In contrast, efforts to minimise the impacts of urban developments in marine environments have been far more restricted in their extent and scope, and have often overlooked the ecological role of the built environment as potential habitat. Urban foreshore developments, i.e. those built on the interface of intertidal and/or subtidal zones, have the potential to incorporate clear multi-functional outcomes, by supporting novel ecosystems. We present a step-by-step eco-engineering framework for building blue that will allow coastal managers to facilitate planning and construction of sustainable foreshore developments. Adopting such an approach will incorporate ecological principles, thereby mitigating some of the environmental impacts, creating more resilient urban infrastructure and environments, and maximising benefits to the multiple stakeholders and users of marine urban waterfronts.

Range and habitat associations of the native macroalga Caulerpa filiformis in New South Wales, Australia

Caulerpa filiformis is a green seaweed found in New South Wales (NSW, Australia), South Africa, Mozambique and Peru. It has been suggested that the abundance of the species has increased in NSW over recent decades. Extensive aerial and diver surveys identified a 500-km northerly extension to the range of C. filiformis in NSW (to 28°21′S) compared with previous records. The alga has a disjunct distribution with small isolated populations around rocky headlands in far northern NSW, but then no apparent populations for 350km southwards. The far northern populations could be the result of recent human-mediated transport (a species introduction), or were simply not detected previously. The increased distribution around the previous northerly limit is likely a natural range expansion. The distribution of C. filiformis in NSW and globally seems confined to a temperature range of ~16–23°C. We found no relationship between abundance of C. filiformis and human population or oceanic chlorophyll-a (a surrogate for nutrient availability). We demonstrate that C. filiformis is predominately subtidal, being found along sections of coastline where there is a mixture of rocky reefs and beaches. It is argued that sand movement may have facilitated increases in abundance of C. filiformis.

Mechanisms Influencing the Spread of a Native Marine Alga

Like invasive macrophytes, some native macrophytes are spreading rapidly with consequences for community structure. There is evidence that the native alga Caulerpa filiformis is spreading along intertidal rocky shores in New South Wales, Australia, seemingly at the expense of native Sargassum spp. We experimentally investigated the role physical disturbance plays in the spread of C. filiformis and its possible consequences for Sargassum spp. Cleared patches within beds of C. filiformis (Caulerpa habitat) or Sargassum spp. (Sargassum habitat) at multiple sites showed that C. filiformis had significantly higher recruitment (via propagules) into its own habitat. The recruitment of Sargassum spp. to Caulerpa habitat was rare, possibly due in part to sediment accretion within Caulerpa habitat. Diversity of newly recruited epibiotic assemblages within Caulerpa habitat was significantly less than in Sargassum habitat. In addition, more C. filiformis than Sargassum spp. recruited to Sargassum habitat at some sites. On common boundaries between these two macroalgae, the vegetative growth of adjacent C. filiformis into cleared patches was significantly higher than for adjacent Sargassum spp. In both experiments, results were largely independent of the size of disturbance (clearing). Lastly, we used PAM fluorometry to show that the photosynthetic condition of Sargassum spp. fronds adjacent to C. filiformis was generally suppressed relative to those distant from C. filiformis. Thus, physical disturbance, combined with invasive traits (e.g. high levels of recruitment and vegetative growth) most likely facilitate the spread of C. filiformis, with the ramifications being lower epibiotic diversity and possibly reduced photosynthetic condition of co-occurring native macrophytes.

The timing of Carcinus maenas recruitment to a south-east Australian estuary differs to that of native crabs

Strong seasonal trends in reproduction and early development of many invasive species are commonplace and may differ between introduced and native ranges, reflecting differences in abiotic conditions that trigger reproduction, or in selective pressures. The invasive crab Carcinus maenas has been present in south-east Australia for over 100xa0years, but little is known about its recruitment to benthic substrates in this introduced range. This study assessed the timing of C. maenas and native crab recruitment to Merimbula Lake (36.89oS, 149.92oE) south-eastern Australia between August 2011 and October 2013. It also assessed the effectiveness of four different types of recruitment bags for detecting the invasive crab. Carcinus maenas recruited in greater numbers to bags that contained live oysters than those with oyster shell, artificial turf or without structure. Recruitment of C. maenas peaked in the late winter and spring, while recruitment of most native species peaked in autumn. The timing of C. maenas recruitment contrasted its native European range where recruitment typically occurs in summer and autumn. Although likely triggered by the warmer water temperatures of south-eastern Australia, this differing reproductive phenology of C. maenas between its native and Australian range may also modify its interactions with native crab recruits.

Artificial structures alter kelp functioning across an urbanised estuary

Assessments of human impacts on natural habitats often focus on the abundance of component species, yet physiological and/or sub-lethal effects of stressors on functional attributes may be equally important to consider. Here we evaluated how artificial structures, an integral part of urbanisation in the marine environment, affects key functional properties of the habitat-forming kelp Ecklonia radiata. Given that stressors rarely occur in isolation, we assessed the effects of infrastructure across an urbanised estuary. Estuaries are ideal for studying how multiple anthropogenic and natural stressors influence potential impacts of infrastructure on habitat-forming species because these habitats usually face a wide range and levels of stressors. Here, we compared the abundance of habitat-forming macro-algae as well as the growth, erosion and photosynthetic activity of kelp in artificial and natural habitats across one of the largest urbanised estuaries in the word - Sydney Harbour. We predicted that effects of artificial structures on functional attributes of kelps would be stronger in the inner area of the Harbour, characterised by higher levels of human impacts and low flushing. Contrary to our predictions, we found that effects of infrastructure were consistent across the estuary, regardless of the ecological footprint caused by human activities or natural environmental gradients. When differences were observed between areas of the estuary, they mostly occurred independently of impacts of substrate type. Importantly, we found lower erosion rates of kelp on pilings than on reefs, likely resulting in lower production of detritus in estuaries where natural reefs are degraded or lost and pilings added. Such impacts have important implications for the connectivity among coastal habitats and secondary productivity in adjacent and remote habitats, which are highly dependent on the exportation of kelp detritus. Our study is the first to assess potential functional consequences of urbanisation through physiological and/or biomechanical effects on habitat-formers, an often overlooked mechanism of environmental impact on ecosystem functioning.

Habitat associations of an expanding native alga

There are many examples of native macrophytes becoming locally dominant and spreading outside their traditional distributions, but the causes and impacts are often not understood. In New South Wales, Australia, the green alga Caulerpa filiformis is undergoing a range expansion and has transitioned from a subdominant to a dominant alga on several rocky shores around the Sydney coastline. Here we investigated relationships between established patches of C.xa0filiformis, the habitat it occupies and associated algal communities at multiple subtidal sites over the green algas 700xa0km range. We tested the following predictions: 1) C.xa0filiformis cover differs among substrata, being greatest on turf-forming algae; 2) C.xa0filiformis cover is positively related to environmental variables linked to increased sedimentation (e.g. reduced reef width, surface slope, increased rugosity and distance from shore); 3) occurrence of C.xa0filiformis is associated with a change in macrophyte community structure and a reduction of macrophyte richness; 4) intact native algal canopies inhibit C.xa0filiformis spread, but turf-forming algae and bare sand are susceptible to invasion. Substratum associations were highly consistent among sites, but contrary to our prediction, C.xa0filiformis was most commonly associated with rock or rockxa0+xa0sand substratum and less frequently associated with turf-forming algae substratum. C.xa0filiformis cover was negatively correlated with reef width, which explained most of the variation observed, although local scale variables distance from shore, reef slope, and water depth were also correlated with C.xa0filiformis cover. Algal diversity and community composition typically differed in the presence of C.xa0filiformis, often with a reduction of algal abundances, in particular Sargassum spp., although results varied among substrata and sites. However, monitoring of borders suggested that C.xa0filiformis does not invade and outcompete undisturbed adjacent canopy-forming algae over a 12 month period. Our results suggest that disturbance processes (possibly linked to sedimentation) acting at the site and quadrat scale are likely important determinants of C.xa0filiformis cover and spread, and hence its potential ecological impacts.