Habitat-forming Species Research Articles

Limited Genetic Connectivity Among Sargassum horneri (Phaeophyceae) Populations in the Chinese Marginal Seas Despite Their high Dispersal Capacity.

Sargassum horneri is a habitat-forming species in the Northwest Pacific and an important contributor to seaweed rafts. In this study, 131 benthic samples and 156 floating samples were collected in the Yellow Sea and East China Sea (ECS) to test the effects of seaweed rafts on population structure and connectivity. Our results revealed high levels of genetic diversity in both benthic and floating samples based on concatenated mitochondrial markers (rpl5-rps3, rnl-atp9, and cob-cox2). Phylogenetic analyses consistently supported the existence of two lineages (lineages I and II), with divergence dating to c. 0.692Mya (95% HPD: 0.255-1.841 Mya), indicating that long-term isolation may have occurred during the mid-Pleistocene (0.126-0.781Mya). Extended Bayesian skyline plots demonstrated a constant population size over time in lineage I and slight demographic expansion in lineage II. Both lineages were found in each marginal sea (including both benthic and floating samples), but PCoA, FST , and AMOVA analyses consistently revealed deep genetic variation between regions. Highly structured phylogeographic pattern supports limited genetic connectivity between regions. IMA analyses demonstrated that asymmetric gene flow between benthic populations in the North Yellow Sea (NYS) and ECS was extremely low (ECS→NYS, 2Nm=0.6), implying that high dispersal capacity cannot be assumed to lead to widespread population connectivity, even without dispersal barriers. In addition, there were only a few shared haplotypes between benthic and floating samples, suggesting the existence of hidden donors for the floating masses in the Chinese marginal seas.

Facilitation of an invader by a native habitat-former increases along interacting gradients of environmental stress.

Native habitat-forming species can facilitate invasion by reducing environmental stress or consumer pressure. However, the intensity of one stressor along a local gradient may differ when expanding the scale of observation to encompass major variations in background environmental conditions. In this study, we determined how facilitation of the invasive porcelain crab, Petrolisthes elongatus, by the native tube-forming serpulid, Galeolaria caespitosa, varied with environmental gradients at local (tidal height) and larger (wave exposure) spatial scales. G.caespitosa constructs a complex calcareous matrix on the underside of intertidal boulders and we predicted that its positive effects on P.elongatus density would increase in intensity with shore height and be stronger at wave-sheltered than wave-exposed locations. To test these predictions, we conducted two experiments. First, we determined the effects of serpulid presence (boulders with live or dead serpulid matrix vs. bare boulders) at six shore heights that covered the intertidal distribution of P.elongatus. Second, we determined the effects of serpulid presence (present vs. absent), shore height (high vs. low) and wave exposure (sheltered vs. exposed) on crabs across six locations within the invaded range in northern Tasmania, Australia. In Experiment 1, the presence of serpulids (either dead or alive) enhanced P.elongatus densities at all shore heights, with facilitation intensity (as determined by a relative interaction index; RII) tending to increase with shore height. In Experiment 2, serpulids facilitated P.elongatus across shore heights and wave exposures, although crab densities were lower at high shore levels of wave-sheltered locations. However, the intensity of crab facilitation by serpulids was greater on wave-sheltered than on wave-exposed shores, but only at the high shore level. This study demonstrates that local effects of native habitat-formers on invasive species are dependent on prevailing environmental conditions at larger spatial scales and that, under more stressful conditions, invaders become increasingly reliant on positive interactions with native habitat-formers. Increased strength of local-scale facilitation by native species, dampening broader scale variations in environmental stressors, could enhance the ability of invasive species to establish self-sustaining populations in the invaded range.

Invasive green algae in a western Mediterranean Marine Protected Area: interaction of photophilous sponges with Caulerpa cylindracea

AbstractWe report on the relationships between some conspicuous Mediterranean photophilous sponge species and Caulerpa cylindracea, a non-indigenous species. A diversification of defence strategies and behaviour is highlighted in target species belonging to different orders of Demospongiae from a western Mediterranean Marine Protected Area (NW Sardinian Sea). Caulerpa cylindracea displays a strongly invasive behaviour during body colonization of the Irciniidae Sarcotragus spinosulus and Ircinia retidermata (order Dictyoceratida). These sponges possess pre-adaptive defensive morpho-functional and physiological traits enabling them to partly withstand algal invasion. Also Aplysina aerophoba (order Verongiida) seems to be able to control colonization. Successful anti-Caulerpa strategies characterize the rarely affected Crambe crambe (order Poecilosclerida). Species-specific competitive strategies are displayed at different levels of body architecture, behaviour and physiology by native sponge species. The invasion patterns on sponges, the invasion dynamics in 2016–2017 and topographic distribution of C. cylindracea on S. spinosulus confirm this algal species as a threat, with potential long-term effects on sponge assemblages. Data suggest other kinds of poorly investigated synergic stressors affecting these habitat-forming species. Defence strategies of sponge species take the form of: (1) passive deterrence by morpho-functional pre-adaptive traits as growth form, biomass amount, surface traits, and microhabitat within the sponges' aquiferous system; (2) active physiological defence, whereby the morphology/anatomy of the sponge body is adapted to control invaders, by body remodelling and regenerative processes within the aquiferous system and at the sponge surface; (3) presumed active chemical defence by exudation processes of bioactive compounds.

Marine deforestation leads to widespread loss of ecosystem function.

Trophic interactions can result in changes to the abundance and distribution of habitat-forming species that dramatically reduce ecosystem functioning. In the coastal zone of the Aleutian Archipelago, overgrazing by herbivorous sea urchins that began in the 1990s resulted in widespread deforestation of the region's kelp forests, which led to lower macroalgal abundances and higher benthic irradiances. We examined how this deforestation impacted ecosystem function by comparing patterns of net ecosystem production (NEP), gross primary production (GPP), ecosystem respiration (Re), and the range between GPP and Re in remnant kelp forests, urchin barrens, and habitats that were in transition between the two habitat types at nine islands that spanned more than 1000 kilometers of the archipelago. Our results show that deforestation, on average, resulted in a 24% reduction in GPP, a 26% reduction in Re, and a 24% reduction in the range between GPP and Re. Further, the transition habitats were intermediate to the kelp forests and urchin barrens for these metrics. These opposing metabolic processes remained in balance; however, which resulted in little-to-no changes to NEP. These effects of deforestation on ecosystem productivity, however, were highly variable between years and among the study islands. In light of the worldwide declines in kelp forests observed in recent decades, our findings suggest that marine deforestation profoundly affects how coastal ecosystems function.

Nitrogen sufficiency enhances thermal tolerance in habitat-forming kelp: implications for acclimation under thermal stress

Local and global changes associated with anthropogenic activities are impacting marine and terrestrial ecosystems. Macroalgae, especially habitat-forming species like kelp, play critical roles in temperate coastal ecosystems. However, their abundance and distribution patterns have been negatively affected by warming in many regions around the globe. Along with global change, coastal ecosystems are also impacted by local drivers such as eutrophication. The interaction between global and local drivers might modulate kelp responses to environmental change. This study examines the regulatory effect of NO3− on the thermal plasticity of the giant kelp Macrocystis pyrifera. To do this, thermal performance curves (TPCs) of key temperature-dependant traits–growth, photosynthesis, NO3− assimilation and chlorophyll a fluorescence–were examined under nitrate replete and deplete conditions in a short-term incubation. We found that thermal plasticity was modulated by NO3− but different thermal responses were observed among traits. Our study reveals that nitrogen, a local driver, modulates kelp responses to high seawater temperatures, ameliorating the negative impacts on physiological performance (i.e. growth and photosynthesis). However, this effect might be species-specific and vary among biogeographic regions – thus, further work is needed to determine the generality of our findings to other key temperate macroalgae that are experiencing temperatures close to their thermal tolerance due to climate change.

Species-specific genetic variation in response to deep-sea environmental variation amongst Vulnerable Marine Ecosystem indicator taxa

Understanding the ecological processes that shape spatial genetic patterns of population structure is critical for understanding evolutionary dynamics and defining significant evolutionary and management units in the deep sea. Here, the role of environmental factors (topographic, physico-chemical and biological) in shaping the population genetic structure of four deep-sea habitat-forming species (one sponge - Poecillastra laminaris, three corals - Goniocorella dumosa, Madrepora oculata, Solenosmilia variabilis) was investigated using seascape genetics. Genetic data (nuclear and mitochondrial sequences and microsatellite multilocus genotypes) and environmental variables were employed to build individual-based and population-level models. The results indicated that environmental factors affected genetic variation differently amongst the species, as well as at different geographic scales. For individual-based analyses, different environmental variables explained genetic variation in P. laminaris (dissolved oxygen), G. dumosa (dynamic topography), M. oculata (sea surface temperature and surface water primary productivity), and S. variabilis (tidal current speed). At the population level, factors related to current and food source explained the regional genetic structure in all four species, whilst at the geomorphic features level, factors related to food source and topography were most important. Environmental variation in these parameters may be acting as barriers to gene flow at different scales. This study highlights the utility of seascape genetic studies to better understand the processes shaping the genetic structure of organisms, and to identify environmental factors that can be used to locate sites for the protection of deep-sea Vulnerable Marine Ecosystems.

Bromeliads affect the interactions and composition of invertebrates on their support tree.

Individual species can have profound effects on ecological communities, but, in hyperdiverse systems, it can be challenging to determine the underlying ecological mechanisms. Simplifying species' responses by trophic level or functional group may be useful, but characterizing the trait structure of communities may be better related to niche processes. A largely overlooked trait in such community-level analyses is behaviour. In the Neotropics, epiphytic tank bromeliads (Bromeliaceae) harbour a distinct fauna of terrestrial invertebrates that is mainly composed of predators, such as ants and spiders. As these bromeliad-associated predators tend to forage on the bromeliads' support tree, they may influence the arboreal invertebrate fauna. We examined how, by increasing associated predator habitat, bromeliads may affect arboreal invertebrates. Specifically, we observed the trophic and functional group composition, and the behaviour and interspecific interactions of arboreal invertebrates in trees with and without bromeliads. Bromeliads modified the functional composition of arboreal invertebrates, but not the overall abundance of predators and herbivores. Bromeliads did not alter the overall behavioural profile of arboreal invertebrates, but did lead to more positive interactions in the day than at night, with a reverse pattern on trees without bromeliads. In particular, tending behaviours were influenced by bromeliad-associated predators. These results indicate that detailed examination of the functional affiliations and behaviour of organisms can reveal complex effects of habitat-forming species like bromeliads, even when total densities of trophic groups are insensitive.

Earthquake-driven destruction of an intertidal habitat cascade

Large scale disturbances associated with anthropogenic activities or natural disasters can destroy primary habitat-forming species like corals, seagrasses and seaweeds. However, little research has documented if and on how large-scale disturbances affect secondary habitat formers, such as epiphytes and small animals that depend on biogenic habitats. Here we quantified changes in the abundance of both primary and secondary habitat-forming seaweeds as well as seaweed-associated invertebrates before and after a 7.8 Mw earthquake that uplifted four intertidal reef platforms by 0.5–0.8 m on the Kaikōura coastline in New Zealand. We found that the dominant primary (Hormosirabanksii and three Cystophora species) and secondary (obligate and facultative epiphytes) habitat-forming seaweeds were all decimated and that mobile seaweed-associated animals were significantly less abundant (per gram of seaweed biomass) after the earthquake. Importantly, epiphytes became functionally extinct after the earthquake, as less than 0.1 % of the populations survived, whereas primary habitat formers survived in suitable microhabitats, like water covered tide-pools and tidal channels. Based on these results we also discuss possible cascading ecosystem effects and future scenarios for natural recovery vs. active restoration that could speed up the recovery of habitat-forming species on degraded reefs.

Ecological variables for developing a global deep-ocean monitoring and conservation strategy.

The deep sea (>200 m depth) encompasses >95% of the world's ocean volume and represents the largest and least explored biome on Earth (<0.0001% of ocean surface), yet is increasingly under threat from multiple direct and indirect anthropogenic pressures. Our ability to preserve both benthic and pelagic deep-sea ecosystems depends upon effective ecosystem-based management strategies and monitoring based on widely agreed deep-sea ecological variables. Here, we identify a set of deep-sea essential ecological variables among five scientific areas of the deep ocean: (1) biodiversity; (2) ecosystem functions; (3) impacts and risk assessment; (4) climate change, adaptation and evolution; and (5) ecosystem conservation. Conducting an expert elicitation (1,155 deep-sea scientists consulted and 112 respondents), our analysis indicates a wide consensus amongst deep-sea experts that monitoring should prioritize large organisms (that is, macro- and megafauna) living in deep waters and in benthic habitats, whereas monitoring of ecosystem functioning should focus on trophic structure and biomass production. Habitat degradation and recovery rates are identified as crucial features for monitoring deep-sea ecosystem health, while global climate change will likely shift bathymetric distributions and cause local extinction in deep-sea species. Finally, deep-sea conservation efforts should focus primarily on vulnerable marine ecosystems and habitat-forming species. Deep-sea observation efforts that prioritize these variables will help to support the implementation of effective management strategies on a global scale.

Evidences of fishing impact on the coastal gorgonian forests inside the Portofino MPA (NW Mediterranean Sea)

The gorgonians Paramuricea clavata (Risso, 1826) and Eunicella cavolini (Koch, 1887) are important habitat-forming species within the Mediterranean coralligenous habitats, creating complex, three-dimensional forests that greatly contribute to the underwater landscape of the coastal hard grounds. Dense forests of both species are present within the Portofino Marine Protected Area (Ligurian Sea, NW Mediterranean Sea), showing a distinct bathymetric separation and site-specific differences.Quantitative evidences obtained with a monitoring census conducted in 2016 in 16 sites within the MPA support the existence of a widespread damage on these populations partially due to the mechanical injuries inflicted by recreational and professional demersal fishing gears. Considering all the studied populations, it resulted that 53% of the colonies of P. clavata and 36% of the colonies of E. cavolini were showing traces of impact, of which 10% in P. clavata and 7% in E. cavolini were due to direct entanglements with lost longlines.In the last decades, recurrent massive mortalities related to temperature anomalies and storms, have greatly reduced the gorgonian populations in this area and fishing impact severely affects survived or recovering colonies. Considering the vulnerable status of these species, and the increasing stress due to climate change, a correct management of the anthropic pressures is strongly needed to help preserving these relevant ecosystems.

Anti-herbivory protection by mutualism in marine ecosystems: The case of kelps and hydroids

Plant–herbivore interactions are critical for the functioning of ecosystems, particularly when habitat-forming species are involved. Kelp forests are among the most diverse and productive ecosystems in the world and, along the Chilean coast, they have a high economic importance. Lessonia trabeculata constitutes the main component of the subtidal shallow habitats in Chile. We quantified the consumption of kelp tissues by the snail Tegula tridentata (one of the most important grazers in Chilean kelp habitats), and the modification in this consumption by a guild of epibiotic hydroids growing on kelps. We performed two different sets of experiments with and without the possibility for the snails to select between kelps with hydroids and kelps without them. The consumption by T. tridentata on kelps without hydroids was between 3 and 4 times higher than on kelps with hydroids. Kelp is protected from herbivores by hydroids and can gain nitrogen during low concentration periods of this nutrient in water. Hydroids gain a substratum, and an elevated position above the seafloor where particle capture is facilitated by the effect of kelp in water currents. Predictions using densities of T. tridentate observed in different forests of L. trabeculata along Chilean coasts, show that herbivory pressure can drastically change depending on this density; in forests with high densities, the presence/absence of hydroids could be crucial. Hydroids have shown to be ubiquitous components of the fauna inhabiting the kelp holdfast and forming a rich community in comparison with other epibionts. Despite its low overall biomass, hydroids can be key elements in kelp productivity (affecting, therefore, the community associated to kelps). The kelp-hydroid mutualistic relationship can change the interactions kelp-herbivore.

Taxonomic and functional diversity of mollusk assemblages in a tropical rocky intertidal zone

ABSTRACT We investigated the spatial variation of molluscan assemblages with different habitat-forming species and bare rock habitat in a rocky intertidal zone in northeastern Brazil. The high intertidal zone substrate was covered predominantly of barnacles [Chthamalus bisinuatus (Pilsbry, 1916)], the mid-intertidal of mussels [Brachidontes exustus (Linnaeus, 1758)] and the low intertidal of macroalgae chlorophytes [Gayralia oxysperma (Kützing) K. L. Vinogradova ex Scagel et al., 1989 and Ulva lactuca Linnaeus, 1753], phaeophytes [Sargassum vulgare C. Agardh] and rhodophytes [Palisada flagellifera (J.Agardh) K. W. Nam, 2007]. A total of 3,861 mollusks were recorded, belonging to the classes Gastropoda (9 species; 3,800 individuals), Bivalvia (3 spp.; 54 ind.), and Polyplacophora (1 sp.; 7 ind.). Functional diversity was accessed through the trophic structure, in which we identified food guilds: suspension feeders, grazers, herbivores, and carnivores. The analysis revealed significant differences in mollusk abundance, species richness, diversity indices, and trophic diversity among barnacle belts, mussel beds, algae habitat, and bare rock habitats. The highest species richness and trophic diversity were detected in algae habitat and mussel beds, which showed low abundance. In contrast, barnacle belts registered low species richness and trophic diversity and a high number of individuals. Bare rock recorded low values in all surveyed indices. This result points to the effect of environmental modification caused by habitat-forming species in this system. These species increase environmental complexity and enable the establishment of organisms through facilitation processes. The various food guilds found in this study reaffirm the role of habitat-forming species in providing niches that support different occupation patterns.

Diatoms Dominate and Alter Marine Food-Webs When CO2 Rises

Diatoms are so important in ocean food-webs that any human induced changes in their abundance could have major effects on the ecology of our seas. The large chain-forming diatom Biddulphia biddulphiana greatly increases in abundance as pCO2 increases along natural seawater CO2 gradients in the north Pacific Ocean. In areas with reference levels of pCO2, it was hard to find, but as seawater carbon dioxide levels rose, it replaced seaweeds and became the main habitat-forming species on the seabed. This diatom algal turf supported a marine invertebrate community that was much less diverse and completely differed from the benthic communities found at present-day levels of pCO2. Seawater CO2 enrichment stimulated the growth and photosynthetic efficiency of benthic diatoms, but reduced the abundance of calcified grazers such as gastropods and sea urchins. These observations suggest that ocean acidification will shift photic zone community composition so that coastal food-web structure and ecosystem function are homogenised, simplified, and more strongly affected by seasonal algal blooms.

Climate-driven shift in coral morphological structure predicts decline of juvenile reef fishes.

Rapid intensification of environmental disturbances has sparked widespread decline and compositional shifts in foundation species in ecosystems worldwide. Now, an emergent challenge is to understand the consequences of shifts and losses in such habitat-forming species for associated communities and ecosystem processes. Recently, consecutive coral bleaching events shifted the morphological makeup of habitat-forming coral assemblages on the Great Barrier Reef (GBR). Considering the disparity of coral morphological growth forms in shelter provision for reef fishes, we investigated how shifts in the morphological structure of coral assemblages affect the abundance of juvenile and adult reef fishes. We used a temporal dataset from shallow reefs in the northern GBR to estimate coral convexity (a fine-scale quantitative morphological trait) and two widely used coral habitat descriptors (coral cover and reef rugosity) for disentangling the effects of coral morphology on reef fish assemblages. Changes in coral convexity, rather than live coral cover or reef rugosity, disproportionately affected juvenile reef fishes when compared to adults, and explained more than 20% of juvenile decline. The magnitude of this effect varied by fish body size with juveniles of small-bodied species showing higher vulnerability to changes in coral morphology. Our findings suggest that continued large-scale shifts in the relative abundance of morphological groups within coral assemblages are likely to affect population replenishment and dynamics of future reef fish communities. The different responses of juvenile and adult fishes according to habitat descriptors indicate that focusing on coarse-scale metrics alone may mask fine-scale ecological responses that are key to understand ecosystem functioning and resilience. Nonetheless, quantifying coral morphological traits may contribute to forecasting the structure of reef fish communities on novel reef ecosystems shaped by climate change.

A New Distichopora Species (Cnidaria: Stylasteridae) from the Mesophotic Zone of Palau1

Stylasterid corals are widely distributed in all oceans from the intertidal zone to the deep sea. They are habitat-forming species, reaching high densities and establishing a wide range of associations with other taxa. In the last decades, many new deep-water stylasterids have been described and the faunas of several regions of the globe have been revised; however, the tropical Pacific Ocean remains relatively understudied. In this paper, a new mesophotic species of Distichopora, D. cryptostylus, is described from the Republic of Palau, Caroline Islands. The new Distichopora is distinguished from congenerics by its uniquely shaped coenosteal pores, a paucity of dactylostyles, and apical dactylotomes.

Could ocean acidification influence epiphytism? A comparison of carbon-use strategies between Fucus vesiculosus and its epiphytes in the Baltic Sea

Reduced seawater pH due to elevated carbon dioxide (CO2), a process known as ocean acidification (OA), is a globally significant environmental issue. OA is predicted to influence a range of ecosystem processes, but little is known about how changing seawater carbon chemistry could influence the extent and impacts of epiphytism. In the brackish Baltic Sea, increased epiphytism is associated with coastal eutrophication and the potential for OA to interact with this relationship remains unclear. This study focuses on slow-growing perennial algae Fucus vesiculosus—which is one of the most important habitat-forming species in the Baltic Sea—and two of its most common and abundant filamentous epiphytes Ceramium tenuicorne and Pylaiella littoralis. Material for this study was collected from Estonian coastal waters. The aim of the research was to determine which carbon acquisition strategies these species possess, which could indicate how they respond to predicted changes in seawater chemistry due to elevated CO2. Carbon-use strategies in macroalgae were determined by analyzing natural carbon isotope signatures (δ13C), pH drift experiments, and photosynthesis vs. dissolved inorganic carbon (P vs. DIC) curves. Our results showed that although F. vesiculosus and its filamentous epiphytes all possess a carbon concentrating mechanism (CCM), the potential species-specific variation in the CCMs operation will favor C. tenuicorne over F. vesiculosus and P. littoralis in a future high CO2 world.

Anthropogenic impact is negatively related to coral health in Sicily (Mediterranean Sea)

Shallow-water marine organisms are among the first to suffer from combined effects of natural and anthropogenic drivers. The orange coral Astroides calycularis is a shallow-water bioconstructor species endemic to the Mediterranean Sea. Although raising conservation interest, also given its special position within the Dendrophylliidae, information about the threats to its health is scant. We investigated the health status of A. calycularis at five locations in northwestern Sicily along a gradient of cumulative human impact and the most probable origin of the threats to this species, including anthropogenic land-based and sea-based threats. Cumulative human impact appeared inversely related to the performance of A. calycularis at population, colony, and polyp levels. Sea-based human impacts appeared among the most likely causes of the variation observed. The reduction in polyp length can limit the reproductive performance of A. calycularis, while the decrease of percent cover and colony area is expected to impair its peculiar feeding behaviour by limiting the exploitable dimensional range of prey and, ultimately, reef functioning. This endangered habitat-forming species appeared susceptible to anthropogenic pressures, suggesting the need to re-assess its vulnerability status. Creating microprotected areas with specific restrictions to sea-based human impacts could be the best practice preserve these bioconstructions.

Kelp beds as coastal protection: wave attenuation of Ecklonia radiata in a shallow coastal bay.

Coastal protection from erosion and flooding is a significant ecosystem service provided by vegetated marine systems. Kelp beds are a dominant habitat-forming species on temperate reefs worldwide. While they are valued as hotspots of biodiversity, there is a paucity of information that supports their use in nature-based coastal defence. This includes the effectiveness of kelp beds in attenuating waves approaching the shore and how this influences sediment transport. Wave loggers were deployed at paired kelp bed and control (urchin barren) treatments at four sites in Port Phillip Bay, Australia. The significant wave height offshore (exposed side) to onshore (sheltered side) of the treatment were compared to determine wave attenuation. At three sites, the wave attenuation of kelp beds was significantly less than that of the control. This result was consistent across the environmental conditions recorded in this study. At the fourth site, on average there was no significant difference in wave transmission between kelp and control. However, wave attenuation at kelp beds was 10 % greater than the control during periods of northerly winds. We highlight the importance of disentangling the effects of the reef substratum and kelp when evaluating the efficacy of kelp at providing coastal protection. We have highlighted a significant gap in the research on ecosystem services provided by kelp beds. A greater understanding is needed on which kelp species are able to provide coastal protection, and under what conditions. Such future research is essential for providing managers and policy makers with actionable information on sustainable and cost-effective solutions for coastal defence when faced with a changing climate.

Eco-engineering increases habitat availability and utilisation of seawalls by fish

The replacement of natural marine habitats with less structurally complex human infrastructure has been linked to the homogenisation of epibenthic assemblages and associated changes in fish assemblages. To mitigate these impacts, eco-engineering efforts have focussed on increasing the physical and biogenic complexity of artificial structures, in the form of crevices added to seawalls and the seeding of the substrate with habitat-forming organisms such as oysters. While these studies have assessed how these interventions affect epibenthic assemblages, the effect of these strategies on the behaviour, such as feeding and habitat use, of different functional groups of fish (e.g. cryptobenthic and pelagic) remains uncertain. To do this, we manipulated complexity on seawalls by adding concrete tiles with different physical (flat or structured with crevices and ridges) and biogenic (seeding with two common habitat-forming species or naturally recruited fouling) complexities. We assessed pelagic and cryptobenthic fish species composition, abundance, interaction time with the tiles and number of feeding bites on three occasions 8–12 months after deployment. Cryptobenthic fish interacted more with physically complex tiles than flat tiles, regardless of biogenic complexity. In contrast, cryptobenthic fish fed more from flat tiles compared to physically complex tiles, and also appeared to feed more from tiles seeded with oysters. Pelagic fish interacted and fed more from naturally fouled tiles compared to unfouled control tiles, regardless of physical complexity. This study showed that manipulating complexity at the scales used here affects behaviour of fish, but it does not affect fish community. Increasing physical complexity facilitated fish use of seawalls as habitat by providing refuge, while it also hindered fish feeding by providing refuge for their prey. Cryptobenthic fish are important trophic linkages in their ecosystems and we have shown that by changing habitat complexity, we can change the habitat use and feeding activity of these fish, allowing them to fulfil this essential ecosystem role.

Impacts of Suspended Sediment on Nearshore Benthic Light Availability Following Dam Removal in a Small Mountainous River: In Situ Observations and Statistical Modeling

The 2011–2014 removal of two dams from the Elwha River, WA, delivered ~ 19 Mt of sediment to the marine environment, creating an opportunity to study the sensitivity of a coastal ecosystem to large-scale sediment input. Macroalgae, the primary habitat-forming species in the nearshore, disappeared from the region. It was hypothesized that this mortality event was caused by a reduction in benthic light availability due to increased turbidity. To investigate this connection, nearshore processes and benthic light availability were monitored at 7 locations along the 10-m isobath in 2016 and 2017. The primary driver of light attenuation was suspended sediment, with measured chlorophyll-a and CDOM concentrations contributing < 15% to observed attenuation values. A Bootstrap-aggregated Regression Tree was trained to predict attenuation from the in situ data. Light attenuation was impacted by both sediment transport in the river plume, represented in the model by fluvial suspended sediment load and tidal current direction, and subsurface resuspension, represented by wave height and bed shear velocity. The models were used to hindcast light availability during the dam removal. Total daily benthic light availability was below the 1–2 mol photons/m2/day threshold for macroalgae growth consistently in 2013 and seasonally in 2012 and 2014, supporting the hypothesis that reduced light availability caused the mortality event. Light availability increased in 2016–2017 as the annual sediment load decreased, and macroalgae were concurrently observed in the region. Predicting benthic light availability over event, tidal, and seasonal timescales by accounting for both near-surface and subsurface attenuation will improve management strategies designed to limit ecosystem damage during sediment delivery events.