Antagonistic effects of seawalls and urban sedimentation on epilithic algal matrix (EAM)-feeding fishes.

Marine urbanisation often results in the proliferation of artificial coastal defences and heavy sedimentation, adversely impacting coral reef systems in tropical coastal cities. Knowledge of how motile organisms, such as reef fish, respond to novel human-made habitats and high sedimentation is limited. Here, we examine the role of sloping granite seawalls in supporting reef fishes that utilise the epilithic algal matrix (EAM) as a food resource. We surveyed fish assemblages and feeding activities on seawalls and reef flats, and conducted a field experiment to examine the effects of sediment on EAM feeding rates. Seawalls and reef flats supported distinct fish assemblage composition with significantly greater feeding activity on seawalls. However, reduced feeding activity on EAM with elevated sediment loads suggests that urban sedimentation may limit the utility of this novel feeding ground for nearshore communities. These findings illustrate the complexities and interactive effects of anthropogenic changes driven by coastal urbanisation.

Human-engineered hydrodynamic regimes as a driver of cryptic microinvertebrate assemblages on urban artificial shorelines.

Urban shorelines undergo substantial hydrodynamic changes as a result of coastal engineering and shoreline armouring that can alter sedimentation, turbidity, and other factors. These changes often coincide with major shifts in the composition and distribution of marine biota, however, rarely are hydrodynamic-mediated factors confirmed experimentally as the mechanism underpinning these shifts. This study first characterized hydrodynamic-related distribution patterns among epilithic and epiphytic microinvertebrates on urban seawalls in Singapore. We found reduced microinvertebrate abundances and distinct microinvertebrate community structure within benthic turf algae in areas where coastal defences had reduced wave energy and increased sediment deposition, among other hydrodynamic-related abiotic changes. Low-exposure areas also had reduced densities of macroinvertebrate grazers and less dense turf algae (lower mass per cm2) than adjacent high-exposure areas. Using harpacticoid copepods as a model taxon, we performed a reciprocal transplant experiment to discern between the effects of exposure-related conditions and grazing. Results from the experiment indicate that conditions associated with restricted wave energy from shoreline engineering limit harpacticoid population densities, as transplantation to low-exposure areas led to rapid reductions in abundance. At the same time, we found no effect from grazer exclusion cages, suggesting harpacticoids are minimally impacted by exposure-related gradients in gastropod macrograzer densities over short time scales. Given the key role of intertidal microinvertebrates, particularly harpacticoids, in nearshore food webs, we postulate that human-engineered hydrodynamic regimes are an important factor shaping marine ecosystem functioning in urban areas.

Not all artificial structures are created equal: Pilings linked to greater ecological and environmental change in sediment communities than seawalls.

Artificial structures are agents of change in marine ecosystems. They add novel habitat for hard-substrate organisms and modify the surrounding environment. Most research to date has focused on the communities living directly on artificial structures, and more research is needed on the potential impacts these structures have on nearby communities and the surrounding environment. We compared the sedimentary habitat surrounding two types of artificial structures (pilings and seawalls) to sediments adjacent to rocky reefs using a combination of traditional sediment analyses, stable isotope analysis, and environmental DNA. Artificial and natural shore sediments were best differentiated by sediment variables strongly associated with flow speed. Pilings sediments had significantly finer grain size, higher organic content, and generally lower C:N ratios than sediments adjacent to the other habitat types, suggesting flow is reduced by pilings. Sedimentary assemblages near pilings were also consistent with those predicted under low-flow conditions, with elevated bacterial colonization and increased relative abundances of small deposit feeders compared with other habitat types. Additionally, lumbrinerid polychaetes in pilings sediments had reduced δ15N values, suggesting different detrital resources and fewer trophic linkages compared with lumbrinerids in other habitats. Woody detritus was greater adjacent to seawalls than to natural rocky shores or pilings. Our findings suggest that artificial structures have the potential to influence adjacent soft sediments through changes to sediment properties that affect infaunal and microbial communities, as well as trophic linkages for some consumers. We hypothesize that this is due to a combination of altered flow, differing detrital subsidies, and differing adjacent land-use among habitat types. Managers should consider the potential for changed sediment properties and ecology when deciding where to build different types of artificial structures. Further manipulative experiments are needed to understand mechanisms of change and help manage the impacts of artificial structures on the seafloor.

Urban coral reefs: Degradation and resilience of hard coral assemblages in coastal cities of East and Southeast Asia.

Given predicted increases in urbanization in tropical and subtropical regions, understanding the processes shaping urban coral reefs may be essential for anticipating future conservation challenges. We used a case study approach to identify unifying patterns of urban coral reefs and clarify the effects of urbanization on hard coral assemblages. Data were compiled from 11 cities throughout East and Southeast Asia, with particular focus on Singapore, Jakarta, Hong Kong, and Naha (Okinawa). Our review highlights several key characteristics of urban coral reefs, including "reef compression" (a decline in bathymetric range with increasing turbidity and decreasing water clarity over time and relative to shore), dominance by domed coral growth forms and low reef complexity, variable city-specific inshore-offshore gradients, early declines in coral cover with recent fluctuating periods of acute impacts and rapid recovery, and colonization of urban infrastructure by hard corals. We present hypotheses for urban reef community dynamics and discuss potential of ecological engineering for corals in urban areas.