Presence Of Macroalgae Research Articles

Comparative analysis of seagrass habitat structure: distinct patterns at semi-enclosed and open sites in South Sulawesi, Indonesia

Habitat structure in the seagrass ecosystems are crucial for coastal biodiversity and substrate stability. The purpose of this research conducted from June to December 2021 in South Sulawesi Province, Indonesia was to examine the habitat structure of seagrass beds at two sites with different levels of exposure: Puntondo (semi-enclosed bay) and Batu Kalasi (exposed). Species identification, non-metric multidimensional scaling (nMDS), Bray Curtis cluster analysis, similarity percentage (SIMPER), Particle Size Analysis (PSA), and satellite imagery were used to evaluate complexity, heterogeneity, sediment particle size, and extent. The presence of macroalgae and coral species at Batu Kalasi, likely related to substrate condition, contributed to significant differences in seagrass community composition. The nMDS and Bray Curtis cluster analyses revealed patterns of complexity, while satellite imagery revealed habitat heterogeneity. Seagrass meadow tracking showed a larger and more patchy area in Puntondo Village, measuring 164,318 m², while in Batu Kalasi Island, it only reached 136,901 m². The findings underscore the need for tailored conservation strategies for seagrass ecosystems, in particular to address the specific challenges observed at each site, emphasizing the importance of understanding and preserving diverse coastal environments.

Habitat-forming organisms in the offshore seabed of the western Arabian Gulf

Some macro-benthic organisms, such as macroalgae, bryozoans, sponges, and gorgonian corals, can create heterogeneous frameworks of habitats in the surrounding seabed. Such frameworks are important benthic ecosystems, but knowledge of them is fairly limited in the Arabian Gulf, the hottest sea in the world. In this study, the presence of macroalgae, bryozoans, sponges, and gorgonian corals was discovered in the seabed of the western Arabian Gulf. These organisms had positive co-occurrences with a preference for depths of 20 to 50 m. A hotspot of spatial distribution of macroalgae and bryozoans was also identified, located in the northwestern part of the Arabian Gulf. These habitat-forming organisms were distributed in a patchy pattern with low to moderate levels of density. They were positively associated with several commercial fishes, suggesting their importance to fishery. Our discovery extends the distribution of marine habitat-forming organisms into the offshore waters of the subtropical Arabian Gulf.

Removal of macroalgae from degraded reefs enhances coral recruitment

Declining coral cover on tropical coral reefs often results in a concomitant increase in macroalgae. When proliferation of macroalgae persists outside regular seasonal growth, it can shift the ecosystem dominance away from corals into a permanently altered system. Such an altered system is unlikely to recover naturally, despite ample supply of coral larvae, as coral settlement and survival is reduced by the presence of macroalgae. Physical removal of macroalgae has been proposed to overcome this biotic barrier to recovery, although empirical evidence demonstrating the effects of removal on phase‐shifted reefs is lacking. Here, we manually removed macroalgae from twelve 25 m2 experimental plots (88.5 ± 6.2 kg wet weight per plot; 90% benthic cover decrease) on a degraded reef prior to coral mass spawning across 2 years and recorded the number of coral recruits to settlement tiles and natural substrata. Four months after each spawning event, we found a threefold increase in coral recruits to tiles in plots where macroalgae had been removed (n = 12 plots; February 2019: mean 45.9 ± 12.7 recruits per tile; February 2020: mean 53.9 ± 5.9 recruits per tile) compared to control plots where macroalgae remained (n = 12 plots; February 2019 mean: 13.6 ± 2.8 recruits per tile; February 2020 mean: 17.5 ± 3.5 recruits per tile). These results suggest that, at small scales, macroalgae removal may be a useful intervention to boost recruitment on degraded reefs. Longer‐term monitoring is needed to document if coral survivorship, growth, and subsequent reef recovery occurs.

Seascape Configuration and Fine-Scale Habitat Complexity Shape Parrotfish Distribution and Function across a Coral Reef Lagoon

Structural complexity spanning fine to broad spatial scales can influence the distribution and activity of key organisms within marine ecosystems. However, the relative importance of hard (e.g., corals) and/or soft (e.g., macroalgae) structural complexity for marine organisms is often unclear. This study shows how both broad-scale (seascape configuration of coral structure) and fine-scale habitat complexity (structure height, number of holes, and presence of macroalgae) can influence the abundance and spatial ecology of reef fish. Underwater visual census of fish, surveys of habitats, remote underwater videos, and behavioral observations by following individual fish were used to quantify fine-scale habitat characteristics (e.g., complexity, coral structure height, macroalgae presence) and the abundance, size structure, and behavior (rates of herbivory, tortuosity ratios and total distance travelled) of abundant parrotfish. Both seascape configuration and macroalgae influenced the patterns of fish abundance and rates of herbivory. However, these relationships varied with trophic groups and ontogenetic stages. Abundance of adult and intermediate-phase parrotfishes was positively influenced by densely aggregated coral structures, whereas juvenile abundance was positively influenced by the presence of macroalgae. Foraging path and bite rates of an abundant parrotfish, Chlorurus spilurus, were not influenced by coral structure configuration or height, but the presence of macroalgae increased the bite rates of all juvenile parrotfish. Our results suggest that a combination of seascape configuration, fine-scale habitat complexity, and microhabitat selectivity influence reef fish community structure and foraging behavior, thus altering herbivory. However, these relationships can differ among functional groups of fish and life-history stages. Information on these fish–habitat interactions is critical for identifying habitats that facilitate ecological functions and ensures the successful management and conservation of essential habitats.

Understanding the individual role of fish, oyster, phytoplankton and macroalgae in the ecology of integrated production in earthen ponds

In order to demonstrate that IMTA can be profitable and a good alternative to regular semi-intensive fish mariculture production in earthen ponds three different production treatments with distinct combinations of trophic levels were designed: (i) a combination of fish, filter feeders, phytoplankton and macroalgae, (ii) fish, filter feeders and phytoplankton and (iii) fish, phytoplankton and macroalgae, to evaluate the role of each trophic level within an Integrated Multi-Trophic Aquaculture system (IMTA). Each treatment was carried out under semi-intensive conditions with two replicates, in a total of 6 earthen ponds of 500 m2 surface and depth of 1.5 m. The results showed that fish, oyster, phytoplankton and macroalgae integrated aquaculture is a healthy sustainable production system for mariculture in earthen ponds, providing much more fish supply compared with the other two treatments. Ponds with filter feeders had significantly lower turbidity (Nephelometric Formazin unit (FNU) of 13 in the morning and 17 in the afternoon) when compared to ponds without filter feeders (16 FNU in the morning and 20 FNU in the afternoon) with increased light penetration throughout water column (61 and 55 cm transparency in ponds with filter feeders compared to 51 cm in ponds without filter feeders) and consequently higher photosynthetic activity with significantly higher dissolved oxygen (5.4 mg L−1 in the morning and 6.7 mg L−1 in the afternoon in ponds with filter feeders compared to 5.3 mg L−1 in the morning and 6.4 mg L−1 in the afternoon in ponds without filter feeders) and carbon sequestration (0.50 and 0.53 mg L−1 8 h−1 in ponds with filter feeders and 0.43 mg L−1 8 h−1 in ponds without filter feeders). In the fish, filter feeder, phytoplanton and macroalgae IMTA treatment, phytoplankton played a crucial role because they increased DO levels, removed the excess of nutrients from animal excretion, and was used as food by the filter feeders. Almost as important is the presence of filter feeders since they control the density of the microalgae and particulate matter in the ponds contributing to a more constant level of DO and higher transparency of the water column. The increased transparency and pond fertilization by oyster excretion, resulted in higher proliferation of phytoplankton (chlorophyll a concentrations of 16.5 μg L−1 and 20.2 μg L−1 in ponds with filter feeder and 13.3 μg L−1 in ponds without filter feeder) with benefits not only for filter feeders themselves but also for the macroalgae. At the end there was higher water quality and higher savings (14% day−1) in the energy costs for pond aeration. Meagre, white seabream and flathead grey mullet enhance their performance in IMTA systems with the presence of filter feeders with food conversion rates (FCR) of 1.52 when compared with 2.46 in the regular semi-intensive system composed by fish, phytoplankton and macroalgae. Meagre grew significantly more in IMTA systems with controlled macroalgae while white seabream and flathead grey mullet enhance their performance in the presence of macroalgae. The results show that the fish, oyster, phytoplankton and macroalgae integrated production in earthen ponds is an improved system compared to the regular semi-intensive fish production. The enhanced water quality in these systems leads to improved fish performance and higher biomass production, and to reduction in the energy power used, contributing to greater profitability.

Disentangling the effect of host-genotype and environment on the microbiome of the coral Acropora tenuis.

Genotype-specific contributions to the environmental tolerance and disease susceptibility of corals are widely accepted. Yet our understanding of how host genotype influences the composition and stability of the coral microbiome subjected to environmental fluctuations is limited. To gain insight into the community dynamics and environmental stability of microbiomes associated with distinct coral genotypes, we assessed the microbial community associated with Acropora tenuis under single and cumulative pressure experiments. Experimental treatments comprised either a single pulse of reduced salinity (minimum of 28 psu) or exposure to the cumulative pressures of reduced salinity (minimum of 28 psu), elevated seawater temperature (+2 °C), elevated pCO2 (900 ppm), and the presence of macroalgae. Analysis of 16S rRNA gene amplicon sequence data revealed that A. tenuis microbiomes were highly host-genotype specific and maintained high compositional stability irrespective of experimental treatment. On average, 48% of the A. tenuis microbiome was dominated by Endozoicomonas. Amplicon sequence variants (ASVs) belonging to this genus were significantly different between host individuals. Although no signs of stress were evident in the coral holobiont and the vast majority of ASVs remained stable across treatments, a microbial indicator approach identified 26 ASVs belonging to Vibrionaceae, Rhodobacteraceae, Hahellaceae, Planctomycetes, Phylobacteriaceae, Flavobacteriaceae, and Cryomorphaceae that were significantly enriched in corals exposed to single and cumulative stressors. While several recent studies have highlighted the efficacy of microbial indicators as sensitive markers for environmental disturbance, the high host-genotype specificity of coral microbiomes may limit their utility and we therefore recommend meticulous control of host-genotype effects in coral microbiome research.

Sediment Biogeochemistry of Mesophotic Meadows of Calcifying Macroalgae

Mesophotic (low light) sands were studied in Hawaiian coastal waters (39–204 m water depth) from O‘ahu to Kaho‘olawe by sampling inside and outside of extensive macroalgal meadows of chlorophytes Halimeda kanaloana and Udotea sp. during September 2004, December 2004, and November 2006. Porewater nutrient concentrations in these permeable sediments were comparable to those in nearshore sands and were highly elevated at sediment depths available to holdfasts of some algae (5–10 cm); maximum levels were 3.0 µM reactive phosphorus, 33 µM nitrate, 0.70 µM nitrite, 38 µM ammonium, and 130 µM silicic acid. Benthic material is calculated to be the major source of organic matter driving diagenesis in these sediments. Vegetated sediments appeared more oxidizing than unvegetated sediments, and the presence of macroalgae, particularly Halimeda, was generally associated with higher sediment dissolved inorganic carbon levels. Halimeda-vegetated sediments generally had low dissolved inorganic nitrogen (DIN) levels compared to the Udotea-vegetated and non-vegetated sediments, consistent with the net N loss indicated by sediment stoichiometric relationships. In contrast, Udotea-vegetated sediments showed minimal apparent algal DIN uptake.

Differential larval settlement responses of Porites astreoides and Acropora palmata in the presence of the green alga Halimeda opuntia

Settlement is critical to maintaining coral cover on reefs, yet interspecific responses of coral planulae to common benthic macroalgae are not well characterized. Larval survival and settlement of two Caribbean reef-building corals, the broadcast-spawner Acropora palmata and the planulae-brooder Porites astreoides, were quantified following exposure to plastic algae controls and the green macroalga Halimeda opuntia. Survival and settlement rates were not significantly affected by the presence of H. opuntia in either species. However, ~10 % of P. astreoides larvae settled on the surface of the macroalga, whereas larvae of A. palmata did not. It is unlikely that corals that settle on macroalgae will survive post-settlement; therefore, H. opuntia may reduce the number of P. astreoides and other non-discriminatory larvae that survive to adulthood. Our results suggest that the presence of macroalgae on impacted reefs can have unexpected repercussions for coral recruitment and highlight discrepancies in settlement specificity between corals with distinct life history strategies.

Fish assemblages of a shallow Mediterranean lagoon (Nador, Morocco): An analysis based on species and functional guilds

Background. Due to their location between the continent and the sea and their shallow depths, lagoons play an important ecological role in coastal ecosystems by providing a selection of habitat types for many species, functioning as nursery areas and feeding grounds for marine opportunistic fishes. The fish fauna of the Nador Lagoon (NE Morocco), as in the majority of southern Mediterranean lagoons, has been insufficiently studied, even though the lagoon itself supports important commercial fisheries. The presently reported study investigated the spatial and seasonal uses of shallow habitats for fish fauna in the Nador Lagoon. Materials and methods. Sampling was done using a beach seine net at six stations covering almost entire lagoon and representing habitats differing in many parameters such as the distance from the inlet, bottom sediment characteristics, presence of macroalgae, etc. The fish abundance was standardized as catch per unit effort (CPUE) and the fish community was analysed both in terms of its taxonomic and functional composition (by making use of both ecological and dietary preference guilds). Results. A total of 2872 individuals, representing 15 species from 12 families, was collected during the four seasons. Only six species were always present in all seasons. Fish abundance peaked in spring and the species diversity was higher in spring and summer. The fish assemblage was largely dominated in terms of ecological guild by the resident and benthivorous species. Spatial and seasonal fish assemblages differed both in terms of fish abundance and diversity and also in terms of functional guilds composition. In autumn, the fish assemblage was dominated by the marine juvenile migrants whereas in winter, spring, and summer, resident species were the most abundant. Conclusion. The combination of taxonomic and functional characteristics of the fish assemblages, determined in the presently reported study, may contribute to a better understanding of the ecological networks of the Nador Lagoon, thus providing useful data for its conservation and management.

Macroalgae inhibits larval settlement and increases recruit mortality at Ningaloo Reef, Western Australia.

Globally, many coral reefs are degraded and demonstrate reduced coral cover and increased macroalgal abundance. While negative correlations between macroalgae and coral recruitment have commonly been documented, the mechanisms by which macroalgae affects recruitment have received little attention. Here we examined the effect of macroalgae on larval settlement and the growth and survival of coral recruits, in a field experiment over nine months. Exclusion treatments were used to manipulate herbivory and macroalgal biomass, while settlement tiles measured coral settlement and survival. After nine months the volume of macroalgae was up to 40 times greater in the caged treatments than in controls and the settlement of coral larvae on the undersides of tiles within caged plots was 93% lower than in the uncaged treatments. The growth and survival of coral recruits was also severely reduced in the presence of macroalgae: survival was 79% lower in caged treatments and corals were up to 58% smaller with 75% fewer polyps. These data indicate that macroalgae has an additive effect on coral recruitment by reducing larval settlement and increasing recruit mortality. This research demonstrates that macroalgae can not only inhibit coral recruitment, but also potentially maintain dominance through a positive feedback system.

Direct effects of elevated temperature, reduced pH, and the presence of macroalgae (<I>Dictyota</I> spp.) on larvae of the Caribbean coral <I>Porites astreoides</I>

Through an aquarium-based study, we provide evidence that exposure to macroalgae (Dictyota spp.), seawater with reduced pH (7.6 vs 8.1), and elevated temperature (31.8 vs 28.8 °C) causes distinct and additive repercussions for larval settlement and condition (photochemical efficiency and oxidative stress). Larvae from the common Caribbean coral Porites astreoides (Lamarck, 1816) were provided settlement substrate and exposed to each factor for 72 hrs in isolation and combination under an orthogonal design. Largely, biotic and abiotic factors did not interact in their impact on coral planulae; instead, stressors independently affected the survival, condition, and settlement of P. astreoides. Dictyota spp. and low pH each individually reduced the survival of coral larvae. Furthermore, the presence of Dictyota spp. and elevated temperature distinctly inhibited larval settlement and photochemical efficiency, respectively. When combined, stressors additively increased cellular oxidative damage (lipid peroxidation) approximately four fold compared to larvae maintained under control conditions. The results indicate that each stressor independently impacts distinct and overlapping facets of coral settlement, and suggest that their combined effects could have severe collective consequences for coral demography.

Linking pipefishes and seahorses to their habitat: implications for species conservation in the Venice lagoon

Event Abstract Back to Event Linking pipefishes and seahorses to their habitat: implications for species conservation in the Venice lagoon Piero Franzoi1, Luca Scapin1*, Francesco Cavraro1, Stefano Malavasi1, Federico Riccato2 and Matteo Zucchetta1 1 Ca' Foscari University Venice, Department of Environmental Sciences, Informatics and Statistics (DAIS), Italy 2 Laguna Project snc, Italy Seagrass meadows of the Venice lagoon (Northern Adriatic Sea, Italy) are spatially heterogeneous systems, with attributes such as degree of coverage and plant assemblages varying along multiple environmental gradients. They play a particularly relevant role in conservation of fish diversity, since they provide vital habitats for pipefish and seahorse species (family Syngnathidae). Aim of the study was to investigate the diversity of syngnathid assemblages in the Venice lagoon and link species distributions to habitat characteristics, in order to evaluate the importance of seagrass meadows of the lagoon for the conservation of these species. The analyses of this work are based on eight years of fish data collected between 2001 and 2014 in 186 shallow water sites across the entire lagoon. Water parameters, presence of macroalgae, seagrass presence and coverage, as well as meadows species composition, were considered as predictors in a modelling framework, to explain the distribution of each species. Overall, two species of seahorses and seven species of pipefishes including the Adriatic endemism Syngnathus taenionotus were found. Three species (S. abaster, S. typhle and Nerophis ophidion) were the most abundant, together accounting for 98% of total individuals caught. Both seagrass presence and its coverage resulted significant predictors, with densities of S. abaster, S. typhle, N. ophidion and Hippocampus guttulatus being positively associated with higher percentages of seagrass cover. In addition, some evidences suggested a different functionality of different meadow types in supporting both S. typhle and N. ophidion. The study highlighted the role of Venice lagoon as biodiversity hotspot for syngnathids, since it supports nine out of the 10 species known in the Mediterranean (with only S. phlegon being absent). Results also suggested the importance of habitat management, which should aim at compensating for seagrass habitat loss and therefore helping the conservation of syngnathid diversity in the Venice lagoon. Keywords: Pipefishes, Seahorses, Venice Lagoon, Seagrass meadows, species conservation, Syngnathidae Conference: XV European Congress of Ichthyology, Porto, Portugal, 7 Sep - 11 Sep, 2015. Presentation Type: Poster Presentation Topic: Ecology, Conservation and Invasive Species Citation: Franzoi P, Scapin L, Cavraro F, Malavasi S, Riccato F and Zucchetta M (2015). Linking pipefishes and seahorses to their habitat: implications for species conservation in the Venice lagoon. Front. Mar. Sci. Conference Abstract: XV European Congress of Ichthyology. doi: 10.3389/conf.fmars.2015.03.00115 Copyright: The abstracts in this collection have not been subject to any Frontiers peer review or checks, and are not endorsed by Frontiers. They are made available through the Frontiers publishing platform as a service to conference organizers and presenters. The copyright in the individual abstracts is owned by the author of each abstract or his/her employer unless otherwise stated. Each abstract, as well as the collection of abstracts, are published under a Creative Commons CC-BY 4.0 (attribution) licence (https://creativecommons.org/licenses/by/4.0/) and may thus be reproduced, translated, adapted and be the subject of derivative works provided the authors and Frontiers are attributed. For Frontiers’ terms and conditions please see https://www.frontiersin.org/legal/terms-and-conditions. Received: 16 Nov 2015; Published Online: 22 Nov 2015. * Correspondence: Dr. Luca Scapin, Ca' Foscari University Venice, Department of Environmental Sciences, Informatics and Statistics (DAIS), Venezia, 30170, Italy, luca.scapin@stud.unive.it Login Required This action requires you to be registered with Frontiers and logged in. To register or login click here. Abstract Info Abstract The Authors in Frontiers Piero Franzoi Luca Scapin Francesco Cavraro Stefano Malavasi Federico Riccato Matteo Zucchetta Google Piero Franzoi Luca Scapin Francesco Cavraro Stefano Malavasi Federico Riccato Matteo Zucchetta Google Scholar Piero Franzoi Luca Scapin Francesco Cavraro Stefano Malavasi Federico Riccato Matteo Zucchetta PubMed Piero Franzoi Luca Scapin Francesco Cavraro Stefano Malavasi Federico Riccato Matteo Zucchetta Related Article in Frontiers Google Scholar PubMed Abstract Close Back to top Javascript is disabled. Please enable Javascript in your browser settings in order to see all the content on this page.

Coral-algae metabolism and diurnal changes in the CO2-carbonate system of bulk sea water.

Precise measurements were conducted in continuous flow seawater mesocosms located in full sunlight that compared metabolic response of coral, coral-macroalgae and macroalgae systems over a diurnal cycle. Irradiance controlled net photosynthesis (Pnet), which in turn drove net calcification (Gnet), and altered pH. Pnet exerted the dominant control on [CO32−] and aragonite saturation state (Ωarag) over the diel cycle. Dark calcification rate decreased after sunset, reaching zero near midnight followed by an increasing rate that peaked at 03:00 h. Changes in Ωarag and pH lagged behind Gnet throughout the daily cycle by two or more hours. The flux rate Pnet was the primary driver of calcification. Daytime coral metabolism rapidly removes dissolved inorganic carbon (DIC) from the bulk seawater and photosynthesis provides the energy that drives Gnet while increasing the bulk water pH. These relationships result in a correlation between Gnet and Ωarag, with Ωarag as the dependent variable. High rates of H+ efflux continued for several hours following mid-day peak Gnet suggesting that corals have difficulty in shedding waste protons as described by the Proton Flux Hypothesis. DIC flux (uptake) followed Pnet and Gnet and dropped off rapidly following peak Pnet and peak Gnet indicating that corals can cope more effectively with the problem of limited DIC supply compared to the problem of eliminating H+. Over a 24 h period the plot of total alkalinity (AT) versus DIC as well as the plot of Gnet versus Ωarag revealed a circular hysteresis pattern over the diel cycle in the coral and coral-algae mesocosms, but not the macroalgae mesocosm. Presence of macroalgae did not change Gnet of the corals, but altered the relationship between Ωarag and Gnet. Predictive models of how future global changes will effect coral growth that are based on oceanic Ωarag must include the influence of future localized Pnet on Gnet and changes in rate of reef carbonate dissolution. The correlation between Ωarag and Gnet over the diel cycle is simply the response of the CO2-carbonate system to increased pH as photosynthesis shifts the equilibria and increases the [CO32−] relative to the other DIC components of [HCO3−] and [CO2]. Therefore Ωarag closely tracked pH as an effect of changes in Pnet, which also drove changes in Gnet. Measurements of DIC flux and H+ flux are far more useful than concentrations in describing coral metabolism dynamics. Coral reefs are systems that exist in constant disequilibrium with the water column.

Effects of green macroalgal blooms on the meiofauna community structure in the Bay of Cádiz

The effect of macroalgal blooms on the abundance and community structure of intertidal sediment meiofauna was studied using an in situ enclosure experiments (Bay of Cádiz, Spain). Meiofaunal abundance (3500–41,000ind 10cm−2) was three to sevenfold higher in the presence of macroalgae. Nematoda were the dominant taxon both in Control (52–82%) and Macroalgae plots (92–96%), followed by Harpacticoida Copepoda and Ostracoda. Non-metric Multi-Dimensional Scaling (MDS) analysis clearly separated the meiofaunal community from Control and Macroalgae plots. Organic matter, organic carbon, total nitrogen, chlorophyll a and freeze-lysable inorganic nutrients were higher in Macroalgae plots, and were highly correlated with the horizontal MDS axis separating Control and Macroalgae meiofaunal communities. Meiofaunal abundance and taxonomic composition in the Bay of Cádiz seem to be bottom–up controlled either through a grazer system based on microphytobenthos in bare sediments or through a decomposer system in macroalgae affected sediments.

Temporal shifts in top‐down vs. bottom‐up control of epiphytic algae in a seagrass ecosystem

In coastal marine food webs, small invertebrate herbivores (mesograzers) have long been hypothesized to occupy an important position facilitating dominance of habitat-forming macrophytes by grazing competitively superior epiphytic algae. Because of the difficulty of manipulating mesograzers in the field, however, their impacts on community organization have rarely been rigorously documented. Understanding mesograzer impacts has taken on increased urgency in seagrass systems due to declines in seagrasses globally, caused in part by widespread eutrophication favoring seagrass overgrowth by faster-growing algae. Using cage-free field experiments in two seasons (fall and summer), we present experimental confirmation that mesograzer reduction and nutrients can promote blooms of epiphytic algae growing on eelgrass (Zostera marina). In this study, nutrient additions increased epiphytes only in the fall following natural decline of mesograzers. In the summer, experimental mesograzer reduction stimulated a 447% increase in epiphytes, appearing to exacerbate seasonal dieback of eelgrass. Using structural equation modeling, we illuminate the temporal dynamics of complex interactions between macrophytes, mesograzers, and epiphytes in the summer experiment. An unexpected result emerged from investigating the interaction network: drift macroalgae indirectly reduced epiphytes by providing structure for mesograzers, suggesting that the net effect of macroalgae on seagrass depends on macroalgal density. Our results show that mesograzers can control proliferation of epiphytic algae, that top-down and bottom-up forcing are temporally variable, and that the presence of macroalgae can strengthen top-down control of epiphytic algae, potentially contributing to eelgrass persistence.

Fish Species Distribution in Seagrass Habitats of Chesapeake Bay are Structured by Abiotic and Biotic Factors

AbstractSeagrass habitats have long been known to serve as nursery habitats for juvenile fish by providing refuges from predation and areas of high forage abundance. However, comparatively less is known about other factors structuring fish communities that make extensive use of seagrass as nursery habitat. We examined both physical and biological factors that may structure the juvenile seagrass‐associated fish communities across a synoptic‐scale multiyear study in lower Chesapeake Bay. Across 3 years of sampling, we collected 21,153 fish from 31 species. Silver Perch Bairdiella chrysoura made up over 86% of all individuals collected. Nine additional species made up at least 1% of the fish community in the bay but were at very different abundances than historical estimates of the fish community from the early 1980s. Eight species, including Silver Perch, showed a relationship with measured gradients of temperature or salinity and Spot Leiostomus xanthurus showed a negative relationship with the presence of macroalgae. Climate change, particularly increased precipitation and runoff from frequent and intense events, has the potential to alter fish–habitat relationships in seagrass beds and other habitats and may have already altered the fish community composition. Comparisons of fish species to historical data from the 1970s, our data, and recent contemporary data in the late 2000s suggests this has occurred.Received September 4, 2012; accepted May 5, 2013

Positive and negative effects of habitat-forming algae on survival, growth and intra-specific competition of limpets.

Understanding the effects of environmental change on the distribution and abundance of strongly interacting organisms, such as intertidal macroalgae and their grazers, needs a thorough knowledge of their underpinning ecological relationships. Control of grazer-plant interactions is bi-directional on northwestern European coasts: grazing by limpets structures populations of macroalgae, while macroalgae provide habitat and food for limpets. Scottish shores dominated by the macroalga Fucus vesiculosus support lower densities and larger sizes of limpets Patella vulgata than shores with less Fucus. These patterns may be due to differences in inter-size-class competitive interactions of limpets among shores with different covers of Fucus. To examine this model, densities of small and large limpets were manipulated in plots with and without Fucus. Amounts of biofilm were measured in each plot. The presence of Fucus increased survival but hindered growth of small (15 mm TL) limpets, which were negatively affected by the presence of large limpets (31 mm TL). In contrast, large limpets were not affected by the presence of Fucus or of small limpets. This suggests the occurrence of asymmetric inter-size-class competition, which was influenced by the presence of macroalgae. Macroalgae and increased densities of limpets did not influence amounts of biofilm. Our findings highlight the role of interactions among organisms in generating ecological responses to environmental change.

Carbon and nitrogen dynamics in shallow photic systems: Interactions between macroalgae, microalgae, and bacteria

We tracked carbon (C) and nitrogen (N) uptake into sediments in the presence and absence of benthic macroalgae using dual stable isotope tracers in combination with compound‐specific isotope analyses of hydrolyzable amino acids and phospholipid‐linked fatty acids to quantify the uptake and retention of C and N within bulk sediments, benthic microalgae (BMA), and heterotrophic bacteria. Stable isotope tracers (as 15NH+4 and H13CO−3) were added to mesocosms either via the surface water or pore water for the first 14 d of the 42‐d experiment. Macroalgae and sediments exposed to ambient light and dark cycles rapidly took up label from both sources and retained label for at least 4 weeks after isotope additions ended. BMA dominated sediment uptake of 13C and 15N, initially accounting for 100% of total uptake. Over time, heterotrophic bacterial uptake became relatively more important, increasing from 0% on day 1 to 20–50% on day 42, indicating a close coupling between BMA and bacterial production. In treatments with macroalgae, sediment 13C and 15N uptake was ∼ 40% lower than treatments without macroalgae, likely because of shading of the sediment surface by macroalgae, which decreased BMA production, which in turn decreased bacterial production. Overall, sediments served as a sink for C and N through uptake and retention by the microbial community, but retention was lower in the presence of macroalgae.

Combining environmental gradients to explain and predict the structure of demersal fish distributions

AbstractAim Species distribution modelling was used to investigate relationships between benthic environmental variables and patterns of demersal fish species distributions using fish occurrence data collected from spatially sparse point sampling. Our goals were to determine: (1) the relative importance of different environmental variables, (2) whether accurate predictions can be developed from these models, and (3) how a comparison of the models against known species ecology can be used to assess model relevance and to provide further insights that could drive future studies of these species.Location The Recherche Archipelago, southern Western Australia.Methods Fish distribution data were collected using baited remote underwater video systems (BRUVS): environmental characteristics (substrate type, macroalgal type and presence of sessile biota) were derived from the video footage; and water depth was measured with a depth sounder. Two species distribution modelling techniques were used to explore and quantify the contribution of environmental characteristics to distribution models of 10 temperate marine fish species.Results Substrate type (reef, sand, cobble) was the most influential variable, and water depth and macroalgal type influenced the probable occurrence of species even over the same substrate type. The probable occurrences of all but one fish species were predicted very successfully, with observed presences being predicted correctly with accuracies > 76%. These predictions were possible due to strong associations between these species and benthic features recorded in this study (substrate type, depth and macroalgal type).Main conclusions The results demonstrate that the combined influence of multiple environmental gradients must be considered to further develop our understanding of how the environment structures demersal fish distributions. The species distribution models not only agreed with the known ecology of the species examined in detail, but also provided more information on the strength of each environmental attribute and interactions between attributes. These interactions may have previously been misinterpreted as, for example, a depth‐driven response. Additional drivers of distribution that were previously not considered, such as the presence of macroalgae, were also found to have influence. This information thus provides a better description of a species’ niche, which is of value to both species management and conservation.

Macroalgae Has No Effect on the Severity and Dynamics of Caribbean Yellow Band Disease

By removing herbivores and promoting increases in macroalgae, overfishing is thought to indirectly cause coral disease and mortality. We performed three field manipulations to test the general hypothesis that overfishing and the subsequent alteration of coral reef trophic dynamics are a cause of coral epizootics. Specifically, we asked whether the presence of macroalgae can influence within- and among-colony spread rates of Caribbean Yellow Band Disease in Montastraea faveolata. Macroalgae were placed next to infected and healthy, adult and small coral colonies to measure effects on disease spread rate, coral growth and coral survival. Surprisingly, the addition of macroalgae did not affect disease severity or coral fitness. Our results indicate that macroalgae have no effect on the severity and dynamics of Caribbean Yellow Band Disease and that fisheries management alone will not mitigate the effects of this important epizootic.