Macroalgal Communities Research Articles

Nearshore ecosystems on seabird islands are potentially influenced by invasive predator eradications and environmental conditions: a case study at the Mercury Islands, New Zealand

Seabirds maintain island ecosystem function by providing rich marine-derived nutrients to the islands where they nest. These nutrients are returned to the sea through runoff, fertilizing the nearshore environment. Invasive predators disrupt this bottom-up control by decimating seabird populations. While invasive predator eradications lead to terrestrial recovery on seabird islands, there is little information on the nearshore impact. We determined how nearshore macroalgae communities and seabird-derived nitrogen concentrations are influenced by predator eradications and environmental parameters (sampling depth, season, wave exposure, and runoff). This case study examined 4 islands in the Mercury Islands archipelago, representing 3 eradication histories: never invaded by mammalian predators, eradicated over 30 yr ago, and eradicated 2 yr ago. Macroalgal diversity was highest at never-invaded islands, followed by islands in order of eradication year (eradicated 30 and 2 yr ago). The amount of seabird-derived nitrogen (δ15N) in algae was higher during the rainy season and decreased with sampling depth and wave exposure. Sampling near high runoff points resulted in increased δ15N in red algae alone. Never-invaded islands had the highest δ15N in most species. With species found at both eradicated islands, the recently eradicated island had unexpectedly higher δ15N than the island eradicated over 30 yr ago. This discrepancy may be a result of the recently eradicated island’s large size and presence of streams, estuaries, and sheltered bays. Studying nearshore habitats is crucial in understanding the extent to which seabirds act as a conduit of the land-sea interface and the marine impacts of island management.

Effects of herbivorous gastropod grazing on the sedimentation and succession of subtidal macroalgal assemblages

The subtidal zone on cold temperate rocky coasts is an environment exposed to much less physical disturbance than the intertidal, and sediment deposition is continuous. Removal of this sediment by gastropod grazers is therefore presumed to affect the structure of subtidal algal communities. This concept was investigated by evaluating the grazing effects of the dominant herbivorous gastropod Omphalius rusticus by exclusion experiments in the field. Settlement plates of both exclusion and control treatments were placed every month from November 2014, and immersed for 1 mo. Algae colonized from May and tended to increase in biomass toward summer. No marked differences were observed in the algal composition of exclusion and control. Sediment deposition showed no apparent seasonal changes. Cumulative successional plates of both exclusion and control treatments were placed in November 2014, and immersed for 1 to 9 mo. The colonization of algae started in February and the species number peaked earlier in the exclusion and later in the control. Sediment load and algal biomass were high from February to May in the exclusion, and from April to July in the control. Seedlings of Sargassum confusum were found in both plots starting in July. We conclude that O. rusticus constantly removed sediments by its grazing activity and had a large impact on the formation of the macroalgal community by controlling sediment deposition. Its presence delayed the colonization of early-successional turf algal species, but did not affect colonization of late-successional canopy-forming algae such as S. confusum.

Biomass and species richness relationships in macroalgal communities that span intertidal and subtidal zones

Investigations of the strong environmental gradients within intertidal and subtidal rocky reefs have contributed significantly to our understanding of ecological processes, but studies exploring how algal community structure responds to the extreme environmental transition of the intertidal-subtidal interface are rare. Our objective was to examine patterns in macroalgal distribution and species richness with depth on temperate rocky reefs. Standing algal biomass and richness were measured on 6 representative reefs in southern New Zealand, across 5 depth strata from the high intertidal zone, 1.5 m above mean low water (MLW), to the subtidal zone, 10 m below MLW. We found a unimodal relationship between algal richness and biomass across the depths, where maximum species richness occurred at intermediate levels of biomass. These results are consistent with many terrestrial plant studies across strong environmental gradients. Biomass decreased down the shoreline, with the exception of the high intertidal where the lowest biomass was recorded, whilst species richness increased down the shoreline. Additionally, strong patterns of dominance were observed, with a single species (not always the same species) contributing >56% of the total biomass across all depth strata examined. This dominance could have important implications for ecosystem provisioning across this system, particularly if dominant species are found to be vulnerable to the impacts of local and/or global change. The strong environmental gradients that characterise the intertidal-subtidal transition on rocky reefs over relatively small and experimentally tractable spatial scales enable opportunities to further advance our understanding of the mechanisms controlling the distribution of biodiversity.

Epibenthic Harmful Marine Dinoflagellates from Fuerteventura (Canary Islands), with Special Reference to the Ciguatoxin-Producing Gambierdiscus

The relationship between the ciguatoxin-producer benthic dinoflagellate Gambierdiscus and other epibenthic dinoflagellates in the Canary Islands was examined in macrophyte samples obtained from two locations of Fuerteventura Island in September 2016. The genera examined included Coolia, Gambierdiscus, Ostreopsis, Prorocentrum, Scrippsiella, Sinophysis, and Vulcanodinium. Distinct assemblages among these benthic dinoflagellates and preferential macroalgal communities were observed. Vulcanodinium showed the highest cell concentrations (81.6 × 103 cells gr−1 wet weight macrophyte), followed by Ostreopsis (25.2 × 103 cells gr−1 wet weight macrophyte). These two species were most represented at a station (Playitas) characterized by turfy Rhodophytes. In turn, Gambierdiscus (3.8 × 103 cells gr−1 wet weight macrophyte) and Sinophysis (2.6 × 103 cells gr−1 wet weight macrophyte) were mostly found in a second station (Cotillo) dominated by Rhodophytes and Phaeophytes. The influence of macrophyte’s thallus architecture on the abundance of dinoflagellates was observed. Filamentous morphotypes followed by macroalgae arranged in entangled clumps presented more richness of epiphytic dinoflagellates. Morphometric analysis was applied to Gambierdiscus specimens. By large, G. excentricus was the most abundant species and G. australes occupied the second place. The toxigenic potential of some of the genera/species distributed in the benthic habitats of the Canary coasts, together with the already known presence of ciguatera in the region, merits future studies on possible transmission of their toxins in the marine food chain.

Distribution gradient of fucoid algae (Sargassaceae, Phaeophyta) along the coastline of Okinawa Island, southern Japan: Relationship to environmental factors

Coastal ecosystems can provide significantly high ecosystem services via various processes. Tropical and/or subtropical coastal ecosystems are constituted by reef-building corals, macroalgae, and seagrasses. The study was conducted in a subtropical coastal area, Okinawa Island, Japan, where macroalgal beds are distributed. Since few studies have focused on the macroalgal community compared to corals in the study area, the relationship between macroalgal (especially Sargassaceae species) distribution and environmental factors were evaluated. Environmental parameters used in the analysis were obtained by field observation, geographic analysis, satellite observation of water quality, and numerical simulation of wave height. Sampling was conducted mainly between May and August in 2018 and 2019, and 31 sampling stations were established along the coastline of Okinawa Island. Though hermatypic corals occurred in most of the stations, the distribution of Sargassaceae was spatially skewed from the southern and central part of the east coast. Canonical correspondence analysis (CCA) was applied to obtained environmental variables and presence–absence data on macroalgae (Sargassaceae), corals, and seagrasses. CCA results indicated that the geographical setting (openness) and wave exposure in winter are the primary factors determining the macroalgal distribution, whereas nutrient (dissolved inorganic nitrogen, phosphorous, and silicon) concentrations at each sampling occasion were not significant factors. Since coastal ecosystems are threatened by both local and global impacts, elucidation of the major mechanism limiting Sargassaceae distribution is significant for coastal ecosystem management, here we use Okinawa Island as an example.

Influence of monsoonal storm disturbance on the diversity of intertidal macroalgae along the eastern coast of Johor (Malaysia)

The effects of cyclical disturbances towards macroalgal community structure could reveal the progression of short-term colonisation but are understudied in tropical settings, especially in Malaysia. The storms associated with monsoon events modify the local macroalgal community structure, and this study aims to determine the variation in species, assemblage and cover of macroalgae during the monsoonal cycle of 2015 to 2016. Four locations (Pantai Pasir Lanun, Pulau Mawar, Telok Gorek and Tanjung Lompat) along the eastern coast of Johor were surveyed during the post-monsoon (January–April 2015), pre-monsoon (May–August 2015), monsoon (September–December 2015) and the beginning of the subsequent post-monsoon (January–February 2016) periods. Line transects (25 m) were deployed at the study sites, and along each line, square quadrats (50 × 50 cm) were placed alternately at every 1 m. These transects provided data on macroalgal cover, frequency and diversity indices. Overall, macroalgal species richness was highest at Tanjung Lompat (n = 31) and lowest at Pulau Mawar (n = 5). The physical disturbances associated with the storm event were most profound at Pantai Pasir Lanun and Pulau Mawar. Here, macroalgal cover reduced to 0% at the end of the monsoon period in December 2015. The highest Shannon index value was observed at Tanjung Lompat (1.54) and lowest at Telok Gorek (0.52). Comparisons between the monsoonal periods revealed that the highest Shannon index value was found during the pre-monsoon period of 2015 (up to 2.13) compared with the monsoon period of 2015 (up to 1.49). We report that the species Chaetomorpha spp. and Cladophoropsis membranacea played a dominant role as first-colonisers, appearing within 1 month from the onset of the post-monsoon period. This study suggests that despite a loss of local macroalgal diversity after storm events, natural re-establishment of the macroalgal community would ensue in cyclical patterns.

Correlation between macroalgae diversity and water quality in Southwest Maluku waters

Macroalgae play an important role in the ecosystem of the coastal area, serving as a shelter ground, nursery ground, and feeding ground. Macroalgae communities are directly influenced by water quality. This study aim was to determine the correlation between the macroalgae diversity and water quality in southwest Maluku waters. This research was conducted in September 2019 at seven research stations. Macroalgae samples were collected by transect method, while seawater quality was measured using Van Dorn Water Sampler. The macroalgae diversity, species composition, and dominance were determined. Water quality parameters analyzed were temperature, salinity, pH, phosphate, nitrate, and ammonia. Correlations between macroalgae diversity and water quality were determined using principal component analysis. This study recorded 45 species of macroalgae consisting of 15 species of red algae (Rhodophyta), 6 species of brown algae (Phaeophyta), and 24 species of green algae (Chlorophyta). Diversity Index varied ranged from low to moderate categories (0.969 - 2.345). Water quality in general is still quite good for macroalgae life. Macroalgae diversity and water quality correlate and influence each other.

High-resolution spatial distribution ofpCO2in the coastal Southern Ocean in late spring

AbstractWe present a high-resolution spatial study of ocean surface carbon dioxide partial pressure (pCO2), temperature and salinity coupled with a seismic survey performed in subpolar waters with a variable presence of glaciers along the coastal margins of Admiralty Bay and the Bransfield Strait, northern Antarctic Peninsula, during the late spring season. Three zones were identified in this bay. The shallow and relatively fresh SHALLOW GLACIER THAW zone in the inner portion of the bay had high freshwater inputs from active glacial meltwater channels, representing higherpCO2levels (median ~438 μatm) than the shallow and relatively salty SHALLOW zone without glaciers along the margins and dominated by macroalgae communities at the bottom, which showed relatively lowpCO2levels (median ~371 μatm). The deep and relatively salty CENTRE zone was highly influenced by seawater intrusions from the Bransfield Strait, representing intermediatepCO2levels (median ~397 μatm). The net sea-air CO2fluxes in late spring obtained from the high-resolution surface survey in Admiralty Bay indicate a condition of near neutral air-sea CO2flux, with a median (25–75% interquartile range) value of -0.07 mmol m-2day-1(ranging from -12.21 to +4.33 mmol m-2day-1), contrasting with the slight source to the atmosphere estimated from measurements only in the CENTRE zone. This finding suggests that temperature-sensitive metabolic and physical-chemical processes may cause significant variability in the ocean surface distribution of CO2over short shoreline distances in the northern Antarctic Peninsula.

Application of multiplatform, multispectral remote sensors for mapping intertidal macroalgae: A comparative approach

Abstract Intertidal macroalgal communities are economically and ecologically important and, with a likely increase in anthropogenic pressures, there is need to evaluate and monitor these diverse habitats. Efforts to conserve and sustainably manage these habitats must be underpinned by accurate, cost‐effective, and efficient data collection methods. The high spatial and temporal resolution of unmanned aerial vehicles (UAVs), compared with satellites and aircraft, combined with the development of lightweight sensors, provides researchers with a valuable set of tools to research intertidal macroalgal communities. The ability of multispectral sensors, mounted on a satellite, an aircraft, and a UAV, to identify and accurately map the intertidal brown fucoid Ascophyllum nodosum (Fucales, Ochrophyta) at a site with a low species diversity of macroalgae were compared. Visual analysis confirmed that the spatial resolution of satellite imagery was too coarse to map intertidal macroalgae as it could not capture the fine spatial patterns of the macroalgal community. High‐resolution RGB (colour) imagery, taken during the aircraft and UAV surveys, was used to collect training and reference data through the visual identification and digital delineation of species. Classes were determined based on the level of taxonomic detail that could be observed, with higher levels of taxonomic detail observed in the UAV imagery over the aircraft imagery. Data from both were used to train a maximum‐likelihood classifier (MLC). The UAV imagery was able to more accurately classify a distinct A. nodosum class, along with other macroalgal and substratum classes (overall accuracy, OA, 92%), than the aerial imagery, which could only identify a lower taxonomic resolution of mixed A. nodosum and fucoid class, achieving a lower OA (78.9%). This study has demonstrated that in a coastal site with low macroalgal species diversity, and despite the spectral similarity of macroalgal species, UAV‐mounted multispectral sensors proved the most accurate for focused assessments of individual canopy‐forming species.

Detecting Long Time Changes in Benthic Macroalgal Cover Using Landsat Image Archive

Coastal macroalgae worldwide provide multiple ecological functions and support vital ecosystem services. Thereby, it is important to monitor changes in the extent of benthic macroalgal cover. However, as in situ sampling is costly and time-consuming, areal estimates of macroalgal species cover are often based only on a limited number of samples. This low sampling effort likely yields very biased estimates, as macroalgal communities are often characterized by large spatial variability at multiple spatial scales. Moreover, ecological time series are often short-term, making it impossible to assess changes in algal communities over decades and relate this to different human pressures and/or climate change. The Landsat series satellites have operated for 40 years. In the current study, we tested if the Landsat sensors could be used for mapping the cover of shallow water benthic macroalgae. This study was carried out at two sites in the West Estonian Archipelago, in the northeastern Baltic Sea. Our results show that the Landsat imagery accurately reflected both spatial and temporal variability in benthic algal cover. To conclude, the current methodology can be used to improve the existing assessments of areal macroalgal cover, or to estimate the cover values, in areas and times lacking ecological observations.

UAV-mounted hyperspectral mapping of intertidal macroalgae

Intertidal macroalgal communities mark the boundary of the marine realm and are faced with many direct and indirect anthropogenic pressures. The effective and sustainable management of these resources must be underpinned by accurate, efficient and cost-effective environmental data collection. Traditional field survey methods, whilst accurate, are time-consuming and limited in the area that can be covered. Remote sensing permits large areas to be rapidly surveyed but the effectiveness of satellites and aircraft for mapping fine-scale intertidal macroalgal mapping is limited by their coarse spatial resolution and restricted operational flexibility. The rapid development of unoccupied aerial vehicle (UAV) and sensor technology can address these issues and provide a potential alternative to established remote sensing platforms. Here, a detailed methodology is presented for the assessment of the commercially and ecologically important intertidal brown macroalga Ascophyllum nodosum using a multirotor UAV and pushbroom hyperspectral sensor. Two different classifiers, Maximum Likelihood Classifier (MLC) and Spectral Angle Mapper (SAM), were compared along with two different sources of spectral profiles, one collected in-situ with a spectral radiometer and the other derived from hyperspectral imagery. Of the classifiers compared, both trained using image-derived spectra, MLC more accurately classified A. nodosum, and other common intertidal species and substratum (Overall Accuracy (OA) 94.7%) than SAM (OA 81.1%). In addition, SAM, trained using in-situ spectra, was the least accurate of the three classifier workflows used (OA 71.4%). The low accuracy of the spectral radiometer approach was likely due to high levels of noise present in the hyperspectral data, a result of the relative instability of the UAV platform causing vibration. The accurate mapping of non-target species also highlights the applicability of this methodology for a broader range of intertidal macroalgal species and communities. This research clearly demonstrates the potential of UAV-mounted hyperspectral remote sensing for mapping the spatially and spectral complex macroalgal habitats found within the intertidal zone.

Long-term response of Dictyota dichotoma var. intricata (C. Agardh) Greville (Phaeophyceae) to ocean acidification: Insights from high pCO2 vents

The ocean acidification may severely affect macroalgal communities of the shallowest coastal habitats where they play relevant structural and functional roles. In this paper were investigated the physiological traits of two populations of Dictyota dichotoma var. intricata, living at two different pH for several generations to assess the reasons of the algae reduced abundance at current (8.1) compared to low pH (6.7). Besides, through transplant experiments, the two populations were analysed for the stress response and reversibility of physiological performance at different pH. The long-term acclimation to high pCO2/low pH favours an ecotype characterised by low energetic costs, higher photochemical efficiency and more resistance to the oxidative stress, compared to individuals living at current pH. These traits promoted the growth and reproduction of the community living at pH 6.7, favouring a lower macroalgal diversity, but a higher ecological success under ocean acidification. The similar behaviour observed between Dictyota living at pH 6.7 and transplanted thalli from pH 6.7 to 8.1, suggested a high tolerance to pH changes in the short-term. On the contrary, adaptive responses may have favoured molecular adjustments on the long-term, as showed by the significant differences between the wild populations at pH 8.1 and 6.7. The overall data indicate that both plasticity and adaptive mechanisms may be the reasons for the success of the brown seaweeds under future high pCO2/low pH. The plasticity due to photochemistry adjustments is likely involved in the early response to environmental changes. Conversely, modifications in the photosynthetic biochemical machinery suggest that more complex adaptive mechanisms occurred in the current population of Dictyota living at pH 6.7. Further studies on population genetics will reveal if any differentiation is taking place at the population level or a local adaptation has already occurred in Dictyota and other brown algae under chronic low pH.

Regional variation in δ13C of coral reef macroalgae

AbstractMacroalgae are important components on coral reefs that underpin food webs but are also indicators of declines in coral reef condition. The sensitivity of macroalgal communities to environmental factors, including climate and pollutants from the land, is important for the future state of coral reefs. We assessed regional variation in the photosynthetic physiology of macroalgae on the Great Barrier Reef and Moreton Bay, Queensland, Australia, using δ13C, tissue chemistry, and metabolic indicators over broad environmental gradients from six sites spanning 13° of latitude and varying distance from shore. Our data set included 568 samples from 54 genera and 19 macroalgal orders. The δ13C of tissues had a strong taxonomic basis (explaining 66% of the variation in δ13C), varying significantly among algal orders. Particularly low values of δ13C, indicating direct use of CO2(aq) in photosynthesis, were mainly associated with lineages within the Rhodophyta. We observed declines in δ13C with increasing depth (inshore only) and latitude, from offshore to inshore sites and with increasing tissue N and N : P ratio. Variation in δ13C of tissues among macroalgal lineages suggests evolution of a range of CO2 acquisition pathways. Patterns in δ13C availability are most consistent with higher availability of CO2(aq) in cooler water and inshore sites. δ13C in macroalgae could provide an important tool for monitoring changes in CO2(aq) with increasing atmospheric CO2, with changes in ocean circulation and with changes to runoff from the land to the coastal oceans.

Evaluation of the Quality of Coastal Ecosystems in the Spermonde Archipelago Using Macroalgae as Indicators

The quality of water in Spermonde Archipelago, South Sulawesi has decreased as a result of high human activities and land use, this can have an impact on the life of marine biota. Macroalgae is suitable for bioindicators because they live in a sessile manner, can accumulate metals and distributed widely. This research aimed to determine coastal ecosystem quality by using macroalgae as indicators on five islands that had different anthropogenic stresses. The research was carried out in April 2019 on Barrang Caddi, Bonebatang, Barrang Lompo, Kodingareng Keke, and Badi Islands. Community structures observed included taxa richness, diversity, evenness, dominance of macroalgae and physicochemical parameters (temperature, turbidity, pH, salinity, DO, BOD, H 2 S, oil, grease, total Phosphate, and nitrate) as well as the measurement of the ecological condition using Quality of Rocky Bottoms (CFR) index. The results showed that physicochemical parameters (temperature, turbidity, pH, salinity, DO, H 2 S, oil, grease, total Phosphate) of the water still met the quality standards of sea water based on Decree of the State Minister for Environment No. 51/2004 for marine biota and other standards for macroalgae growth. BOD and nitrate have exceeded sea water quality standards for marine biota (20 mg/L for BOD and 0.008 mg/L for nitrates). A total of nine species of macroalgae were found in Barrang Caddi and Badi Island, three species in Bonebatang, and five species in Kodingareng Keke and Barrang Lompo Island. The result of Principal Component Analysis (PCA) and Biplot showed that physicochemical water quality has an influence on the macroalgae community structure. While the CFR index shows Barrang Caddi and Badi Island have poor ecological conditions and Bonebatang, Barrang Lompo, and Kodingareng Keke have bad ecological condition.

Trophic plasticity in the sea urchin Paracentrotus lividus, as a function of resource availability and habitat features

Factors controlling herbivory pressure are of central importance in shaping the seascape. In the Mediterranean, the sea urchin Paracentrotus lividus is considered as a keystone herbivore in seagrass meadows and macroalgal communities. Here we explored the trophic behavior of this sea urchin in a shallow seagrass habitat of Cymodocea nodosa mixed with Caulerpa prolifera and interspersed with sandy areas in Alfacs Bay, Ebro Delta (NW Mediterranean). The seasonal pseudo-indigenous bryozoan Amathia verticillata is locally very abundant, and there is also an important population of pen shells Pinna nobilis, providing hard substrate and cover, thus being a unique environment for assessing sea urchin trophic behavior. To this end, we conducted an ensemble of food preference and foraging experiments and stomach content and stable isotope analyses. Our results showed that sea urchins strongly prefer A. verticillata over other local resources, and there was also an important presence of the bryozoan in stomach contents (ca. 44%), coupled with green and decayed seagrass leaves. Stable isotope analyses revealed that over the long term, ca. 65% of the diet of P. lividus was based on decayed seagrass leaves, followed by the bryozoan and green seagrass leaves (21.7 and 13.3%, respectively). The local availability of P. nobilis provides a preferred substrate for sea urchins, which showed limited foraging movements into the surrounding seagrass beds, particularly when A. verticillata was attached to the pen shells. The apparently high contribution of animal and detrital food to P. lividus diet is unprecedented, and suggests an opportunistic feeding behavior in sea urchins in those habitats.

Model simulation of seasonal growth of Fucus vesiculosus in its benthic community

AbstractNumerical models are a suitable tool to quantify impacts of predicted climate change on complex ecosystems but are rarely used to study effects on benthic macroalgal communities. Fucus vesiculosus L. is a habitat‐forming macroalga in the Baltic Sea and alarming shifts from the perennial Fucus community to annual filamentous algae are reported. We developed a box model able to simulate the seasonal growth of the Baltic Fucus–grazer–epiphyte system. This required the implementation of two state variables for Fucus biomass in units of carbon (C) and nitrogen (N). Model equations describe relevant physiological and ecological processes, such as storage of C and N assimilates by Fucus, shading effects of epiphytes or grazing by herbivores on both Fucus and epiphytes, but with species‐specific rates and preferences. Parametrizations of the model equations and required initial conditions were based on measured parameters and process rates in the near‐natural Kiel Outdoor Benthocosm (KOB) experiments during the Biological Impacts of Ocean Acidification project. To validate the model, we compared simulation results with observations in the KOB experiment that lasted from April 2013 until March 2014 under ambient and climate‐change scenarios, that is, increased atmospheric temperature and partial pressure of carbon dioxide. The model reproduced the magnitude and seasonal cycles of Fucus growth and other processes in the KOBs over 1 yr under different scenarios. Now having established the Fucus model, it will be possible to better highlight the actual threat of climate change to the Fucus community in the shallow nearshore waters of the Baltic Sea.

Global-scale species distributions predict temperature-related changes in species composition of rocky shore communities in Britain.

Changes in rocky shore community composition as responses to climatic fluctuations and anthropogenic warming can be shown by changes in average species thermal affinities. In this study, we derived thermal affinities for European Atlantic rocky intertidal species by matching their known distributions to patterns in average annual sea surface temperature. Average thermal affinities (the Community Temperature Index, CTI) tracked patterns in sea surface temperature from Portugal to Norway, but CTI for communities of macroalgae and plant species changed less than those composed of animal species. This reduced response was in line with the expectation that communities with a smaller range of thermal affinities among species would change less in composition along thermal gradients and over time. Local-scale patterns in CTI over wave exposure gradients suggested that canopy macroalgae allow species with ranges centred in cooler than local temperatures ('cold-affinity') to persist in otherwise too-warm conditions. In annual surveys of rocky shores, communities of animal species in Shetland showed a shift in dominance towards warm-affinity species ('thermophilization') with local warming from 1980 to 2018 but the community of plant and macroalgal species did not. From 2002 to 2018, communities in southwest Britain showed the reverse trend in CTI: declining average thermal affinities over a period of modest temperature decline. Despite the cooling, trends in species abundance were in line with the general mechanism of direction and magnitude of long-term trends depending on the difference between species thermal affinities and local temperatures. Cold-affinity species increased during cooling and warm-affinity ones decreased. The consistency of responses across different communities and with general expectations based on species thermal characteristics suggests strong predictive accuracy of responses of community composition to anthropogenic warming.

Snapshot of Macroalgae and Fish Assemblages in Temperate Reefs in the Southern European Atlantic Ecoregion

Most of the biodiversity studies in the South European Atlantic Shelf ecoregion are limited to shallow subtidal or intertidal habitats, while deeper reef habitats, also of relevant ecological importance, are particularly understudied. Macroalgal communities, associated fauna, and sea surface temperature were studied in deep reefs (25–30 m) at two locations in this ecoregion: Parcel, North of Portugal (41° N), and Tarifa, Southern Spain (35° N). Specifically, algal assemblages were assessed using biomass collection and associated ichthyofauna was assessed using visual census techniques using scuba. Seawater surface temperature was higher (>3 °C) in the southern region—Tarifa, compared to the northern region—Parcel. Our survey revealed 18 fish species and 23 algae species. The highest abundance of cold-water species (both macroalgae and fish species) was recorded in Parcel and warm-water species were dominant in Tarifa. In light of climate global trends, both regions might experience biodiversity shifts towards tropicalization. Current knowledge on their biodiversity is imperative to further evaluate potential shifts.

Glacial and depth influence on sublittoral macroalgal standing stock in a high-Arctic fjord

The motivation behind this study is the progression of global warming, which severely affects the entire Arctic and could have effects on macroalgal assemblages. Three sampling sites were located at different distances to glacier fronts to study the effect of glacial exposure on macroalgal community composition and standing stock. In addition, the variability between two depths (5 and 10 m) was investigated. The subtidal communities at each site and depth were dominated by brown macroalgae of the order Laminariales, which made up over 95% of the macroalgal biomass and at least 65% of the macroalgal abundance at each site. The mean macroalgal species richness and abundance were reduced at glacially impacted sites, but the differences were not statistically significant. The community structure based on abundance data varied between the clear and glacially exposed sites due to the higher abundances of Laminaria digitata, Phycodrys rubens and Alaria esculenta at the clear site and Saccharina latissima at the glacial sites. The biomass structure did not vary between sites. The kelp biomass distribution between depths was patchy and dependent on water properties. The maximum biomass was observed at the depth of 10 m at the clear site, while it was observed at shallower depths at the glacial sites. As deglaciation proceeds, new seabed areas are uncovered, and the number of ice-free days grows, creating favourable conditions for macroalgal expansion. However, due to increased turbidity close to the glacier fronts, decreases in species richness, abundance and biomass of macroalgae are likely to occur, as well as the reorganization of the community structure. Such changes may intensify with continued Arctic warming. The hypothesis about reduction of macrophyte abundance and biomass due to the glacial impact was not supported by our data.

Sea urchin control of macroalgal communities across a productivity gradient

Opposing bottom-up ‘resource-driven’ and top-down ‘consumer-driven’ forces interact to shape the structure of ecosystems. While these counteracting forces are well recognised, debate remains regarding which is more influential across space and time. Here we explore bottom-up versus top-down control of macroalgal communities for temperate rocky reef communities in highly urbanised Port Phillip Bay (PPB), southeast Australia. Field surveys show macroalgal cover to paradoxically decline with increasing ‘bottom-up’ nutrient inputs while the abundance of grazing sea urchins (Heliocidaris erythrogramma) increased. The mechanisms underpinning this pattern were examined by constructing urchin-exclusion plots using octocoral (Erythropodium hicksoni) colonies that grow on urchin barren grounds and form natural barriers to grazing sea urchins. Octocoral plots were constructed by cutting 200 mm by 200 mm squares to expose bare reef substratum within the centre of octocoral colonies, which enabled efficient replication of urchin-exclusions on barren grounds across three distinct zones of anthropogenic nutrient input in PPB. Octocoral plots successfully excluded urchins across zones and, in the absence of grazing, macroalgal production increased with increasing nutrient concentration as expected. This novel opportunity to efficiently replicate urchin-exclusions on high-density barren grounds across different zones of ‘bottom-up’ forcing demonstrates that urchin overgrazing can keep pace with and overwhelm increasing macroalgal productivity. Our findings also highlight that impacts of grazing can be greatest where bottom-up forces enable large abundances of herbivores to accumulate, which is counter to perceptions that impacts of herbivores will be greatest where macroalgal productivity is low.