Structure Of Algal Assemblages Research Articles

Does Depth and Sedimentation Interact with Sea Urchins to Affect Algal Assemblage Patterns on Eastern Atlantic Reefs?

A range of factors may affect the composition and abundance of macroalgae on subtidal rocky reefs. We experimentally determined the interactive effect of the occurrence of the long-spine sea urchin, Diadema antillarum, depth and sedimentation levels on macroalgal assemblage structure on eastern Atlantic rocky reefs. Specifically, we manipulated sea urchin densities (removal of all individuals vs. untouched controls at natural densities) on rocky reefs devoid of erect vegetation, and predicted (1) that removal of sea urchins would differently affect macroalgal assemblage structure between deep (16-18 m) and shallow (8-9 m) reef strata, and that (2) the effect of sea urchin removal on macroalgae would be altered under different scenarios of sedimentation (ambient vs. enhanced). Experimental circular plots (2 m in diameter) were set up at 3 locations at Gran Canaria (Canarian Archipelago), and were maintained and monitored every 4 wk for 1 y. At the end of the experimental period, the structure of the algal assemblages differed between urchin treatments and depth strata, with a larger cover of turf and bushlike algae where urchins were removed and at the shallow reef stratum. More important, differences in algal assemblage structure between urchin treatments were irrespective of sedimentation levels, but shifted from the shallow to the deep stratum. This interactive effect was, in turn, observed for bushlike algae, as a result of a larger magnitude of response (i.e., larger cover) in the shallow stratum relative to the deep stratum, but was not detected for either turf or crustose coralline algae. These results highlight the importance of some physical conditions (here, differences in depth) to interact with biotic processes (here, urchin abundance) to create patterns in the organization of subtidal and benthic assemblages.

Community structure and productivity of subtidal turf and foliose algal assemblages

Foliose and turf-forming macroalgae form morphologically distinct assemblages on shallow reefs worldwide and frequently coexist in a patch mosaic. The relative contributions of these algal assemblages to reef productivity and the processes that maintain them, how- ever, have rarely been studied. We found high (80%) species overlap between turfs and foliose macroalgae at Naples Reef off southern California, with differences be- tween the 2 assemblages lying in species dominance and morphology rather than composition. Turfs were domi- nated by a small finely-branched red alga, Pterosiphonia dendroidea, while larger blade-forming red algae, partic- ularly Chondracanthus corymbiferus and Rhodymenia californica, dominated foliose assemblages. Foliose dominants were common in turfs, but were small. Area- specific rates of net primary production of foliose assem- blages were ~3 × higher than those of turfs, while rates of carbon-specific production and turnover times of turfs were ~3 × higher than those of foliose assemblages. These findings suggest that factors other than the growth poten- tial of each assemblage determined their relative bio- mass. Turfs were net heterotrophic, with net community production <0, due to greater abundance of macrofauna, compared to foliose assemblages. The abundance of macrofaunal invertebrates was uncorrelated with pro- ductivity within or among assemblages, suggesting that turfs are structurally better habitats for macrofauna. Our results suggest that maintenance of turf assemblages on Naples Reef is not the result of competitive exclusion of foliose algae by turf species; turf morphology appears to be maintained through the stunting of foliose forms by fishes that crop turfs while foraging for invertebrate prey.

Changes in benthic algal communities following construction of a run-of-river dam

Ecological responses to dam construction are poorly understood, especially for downstream benthic algal communities. We examined the responses of benthic algal communities in downstream reaches of a tributary of the Xiangxi River, China, to the construction of a small run-of-river dam. From February 2003 to August 2006, benthic algae, chemical factors, and habitat characteristics were monitored upstream and downstream of the dam site. This period spanned 6 mo before dam construction and 37 mo after dam construction. Benthic algal sampling yielded 199 taxa in 59 genera that belonged to Bacillariophyta, Chlorophyta, and Cyanophyta. Some physical factors (flow velocity, water depth, and channel width) and 3 algal metrics (diatom species richness, Margalef diversity, and % erect individuals) were significantly affected by the dam construction, whereas chemical factors (e.g., NH4-N, total N, SiO2) were not. Nonmetric multidimensional scaling (NMS) ordinations showed that overall algal assemblage structure downstream of the dam sites was similar to that of upstream control sites before dam construction and for 1 year after dam construction (p > 0.05). However, sites belonging to upstream and downstream reaches were well separated on NMS axis 1 during the 2nd and 3rd years after dam construction. Our results suggest that impacts of dam construction on benthic algal communities took 2 to 3 y to emerge. Further development of a complete set of indicators is needed to address the impact of small-dam construction. Our observations underscore the need for additional studies that quantify ecological responses to dam construction over longer time spans.

Limited impact of Sargassum muticum on native algal assemblages from rocky intertidal shores

Invasive species represent a serious threat to natural ecosystems through a range of negative effects on native species in the region invaded. The invasive species Sargassum muticum has invaded several temperate regions worldwide including the Galician rocky shoreline (northwestern Spain) in Western Europe. The main aim of this study was to assess if colonization by S. muticum has any effect on native algal assemblages by experimental removal of S. muticum. We predicted that in those plots where S. muticum plants were removed, the structure of native algal assemblages would differ from that in plots where S. muticum plants were untouched. In addition, we predicted that the effect of Sargassum removal would be more important than other causes of variability at the small scale investigated. Results indicated limited impact of S. muticum on native assemblages. The impact was only evident on the total number of native taxa and two understory morpho-functional groups, filamentous and foliose algae, rather than on the entire macroalgal assemblages.

Natural densities of mesograzers fail to limit growth of macroalgae or their epiphytes in a temperate algal bed

Summary 1 Herbivory is particularly intense in marine environments, with a higher proportion of primary productivity removed than in terrestrial habitats. Experimental manipulation of large herbivores (fish, urchins) has clearly documented their grazing impacts on algal and seagrass beds. Grazing impacts of mesograzers (small invertebrates such as amphipods and isopods) are, however, less understood due to the practical difficulties in manipulating their abundance in field conditions. 2 We developed a novel technique that successfully manipulated the abundance of herbivorous amphipods on macroalgae without the potential artefacts associated with exclusion cages or mesocosms. We then used this technique to test the effects of reduced amphipod grazing over extended periods on the structure of a temperate algal assemblage. We tested grazer effects on growth rates and epiphyte cover of the brown alga Sargassum linearifolium, and on developing assemblages on bare substrates. 3 Large reductions in the abundance of herbivorous amphipods affected neither the growth rates of S. linearifolium, the cover of its epiphytes, nor the structure of algal assemblages. This result contrasts strongly to previous studies in mesocosms documenting strong impacts of mesograzers on community structure, and we discuss differences in the experimental approaches and biology of the systems that could give rise to the observed differences in grazer impacts. 4 Synthesis. Marine macroalgae and seagrasses support very high densities of small herbivores whose ecological role in these habitats is poorly understood. We have provided the first, replicated experiment that directly manipulates their density in situ to quantify grazer impacts without caging artefacts. Our results indicate that strong impacts are not likely with the naturally occurring amphipod densities in the temperate algal bed studied. Further such experimental tests in field conditions are required to understand the properties of grazer and plant communities that can predict grazer impacts.

Predatory fish loss affects the structure and functioning of a model marine food web

The rate of species loss is increasing on a global scale and predators are most at risk from human‐induced extinction. The effects of losing predators are difficult to predict, even with experimental single species removals, because different combinations of species interact in unpredictable ways. We tested the effects of the loss of groups of common predators on herbivore and algal assemblages in a model benthic marine system. The predator groups were fish, shrimp and crabs. Each group was represented by at least two characteristic species based on data collected at local field sites. We examined the effects of the loss of predators while controlling for the loss of predator biomass. The identity, not the number of predator groups, affected herbivore abundance and assemblage structure. Removing fish led to a large increase in the abundance of dominant herbivores, such as Ampithoids and Caprellids. Predator identity also affected algal assemblage structure. It did not, however, affect total algal mass. Removing fish led to an increase in the final biomass of the least common taxa (red algae) and reduced the mass of the dominant taxa (brown algae). This compensatory shift in the algal assemblage appeared to facilitate the maintenance of a constant total algal biomass. In the absence of fish, shrimp at higher than ambient densities had a similar effect on herbivore abundance, showing that other groups could partially compensate for the loss of dominant predators. Crabs had no effect on herbivore or algal populations, possibly because they were not at carrying capacity in our experimental system. These findings show that contrary to the assumptions of many food web models, predators cannot be classified into a single functional group and their role in food webs depends on their identity and density in ‘real’ systems and carrying capacities.

Effects of the herbivorous minnow, southern redbelly dace (Phoxinus erythrogaster), on stream productivity and ecosystem structure

We used field and mesocosm experiments to measure effects of southern redbelly dace (Phoxinus erythrogaster), a grazing minnow, on stream ecosystem structure and function. Ecosystem structure was quantified as algal filament length, algal biomass, size distribution of particulate organic matter (POM), algal assemblage structure, and invertebrate assemblage structure, whereas ecosystem function was based on gross and net primary productivity. Our experiments showed that moderate densities of Phoxinus temporarily reduced mean algal filament length and mean size of POM relative to fishless controls. However, there was no detectable effect on algal biomass or ecosystem primary productivity. Several factors could explain the lack of effect of Phoxinus on primary productivity including increased algal production efficiency in grazed treatments or increased grazing by other organisms in fishless treatments. The inability of Phoxinus to reduce algal biomass and system productivity contrasts with experimental results based on other grazing minnows, such as the central stoneroller (Campostoma anomalum), and questions the generality of grazer effects in stream ecosystems. However, environmental venue and the spatial and temporal scale of ecosystem measurements can greatly influence the outcome of these experiments.

The relationship between physical variables on topographically simple and complex reefs and algal assemblage structure beneath an Ecklonia radiata canopy

Limestone reefs in South-Western Australia range in topographic complexity at the 1–10 m scale from simple, flat, planar reef structures to topographically complex reefs with many rock walls, overhangs and channels. Algal assemblage structure is known to differ between topographically simple and complex reefs. This research assessed if differences in algal assemblage structure could be associated with the physical environment present on each reef type. Sampling of the physical environment and algal assemblage was carried out on simple, planar reefs and complex, rugose reefs. The physical environment described almost 80% of the algal assemblage structure under an Ecklonia radiata canopy. It was found that topographically simple reefs had greater sediment cover and water depth than adjacent reefs with more complex topography which had more downwelling light, stronger and more turbulent water motion and a greater range of substrate slopes. Greater variation in most physical variables on topographically complex reef shows a greater diversity of microenvironments on that reef type. It is argued that these differences in physical environment between reef types have a considerable influence on algal assemblages by altering key biological processes such as recruitment and interspecific competition.

Effect of fire on benthic algal assemblage structure and recolonization in intermittent streams

Abstract: Dry biofilm on rocks and other substrata forms an important drought refuge for benthic algae in intermittent streams following the cessation of flow. This dry biofilm is potentially susceptible to disturbance from bushfires, including direct burning and/or scorching and damage from radiant heat, particularly when streams are dry. Therefore, damage to dry biofilms by fire has the potential to influence algal recolonization and assemblage structure in intermittent streams following commencement of flow. The influence of fire on benthic algal assemblages and recolonization was examined in intermittent streams of the Grampians National Park, Victoria, Australia, using a field survey and manipulative field experiment. The field survey compared assemblages in two intermittent streams within a recently burnt area (within 5 months of the fire) with two intermittent streams within an unburnt area. The two burnt streams were still flowing during the fire so most biofilms were not likely to be directly exposed to flames. Considerable site‐to‐site and stream‐to‐stream variation was detected during the field survey, which may have obscured potential differences attributable to indirect effects of the fire. The manipulative field experiment occurred in two intermittent streams and consisted of five treatments chosen to replicate various characteristics of bushfires that may influence dry biofilms: dry biofilm exposed directly to fire; dry biofilm exposed to radiant heat; dry biofilm exposed to ash; and two procedural controls. After exposure to the different treatments, rocks were replaced in the streams and algae were sampled 7 days after flow commenced. Differences occurred across treatments, but treatment differences were inconsistent across the two streams. For example, direct exposure to fire reduced the abundance of recolonizing algae and altered assemblage structure in both streams, while radiant heat had an effect on assemblage structure in one stream only. The manipulative field experiment is likely to have represented the intensity of a small bushfire only. Nonetheless, significant differences across treatments were detected, so these experimental results suggest that fire can damage dry biofilms, and hence, influence algal recolonization and assemblage structure in intermittent streams.

Spatial and temporal variation in algal-assemblage structure in isolated dryland river waterholes, Cooper Creek and Warrego River, Australia

The scale at which algal biodiversity is partitioned across the landscape, and the biophysical processes and biotic interactions which shape these communities in dryland river refugia was studied on two occasions from 30 sites in two Australian dryland rivers. Despite the waterholes studied having characteristically high levels of abiogenic turbidity, a total of 186 planktonic microalgae, 253 benthic diatom and 62 macroalgal species were recorded. The phytoplankton communities were dominated by flagellated cryptophytes, euglenophytes and chlorophytes, the diatom communities by cosmopolitan taxa known to tolerate wide environmental conditions, and the macroalgal communities by filamentous cyanobacteria. All algal communities showed significant differences between catchments and sampling times, with a suite of between 5 and 12 taxa responsible for ~50% of the observed change. In general, algal assemblage patterns were poorly correlated with the measured environmental variables. Phytoplankton and diatom assemblage patterns were weakly correlated with several waterhole geomorphic measures, whereas macroalgal assemblage patterns showed some association with variability in ionic concentration.

The ecology of acidification and recovery: changes in herbivore-algal food web linkages across a stream pH gradient

We examined the effects of acidification on herbivore-algal food web linkages in headwater streams. We determined the structure and abundance of consumer and benthic algal assemblages, and gauged herbivory, in 10 streams along a pH gradient (mean annual pH 4.6–6.4). Biofilm taxonomic composition changed with pH but total abundance did not vary systematically across the gradient. Mayflies and chironomids dominated under circumneutral conditions but declined with increasing acidity and their consumption of algae was strongly reduced. Contrary to expectations, several putative shredder species consumed algae, maintaining the herbivore-algal linkage where specialist grazers could not persist. These shifts in functioning could render the communities of acidified streams resistant to reinvasion when acidity ameliorates and water chemistry is restored to a pre-acidification condition. This hypothesis is discussed in the light of recent trends in the chemistry and biology of the UK Acid Waters Monitoring Network sites.

Influence of fishing and functional group of algae on sea urchin control of algal communities in the eastern Atlantic

We address the effect of several factors influencing the depth of interfaces between the shallow photophilic brown algal assemblages and Diadema antillarum urchin-dominated barrens on rocky reefs of the warm-temperate central-eastern Atlantic. By means of a multifactorial observa- tional approach, we hypothesized that it would be locally influenced by (1) the functional structure of the dominant brown algal assemblages (corticated foliose algae versus large frondose coarsely- branched fucoid species), in addition to (2) the fishing status (fished versus unfished sites) and (3) the large-scale topography of the substrate. Our results show that brown algal assemblages reach deeper waters at unfished sites, which is independent of the dominant brown algal functional group. How- ever, monospecific algal facies dominated by large frondose coarsely branched fucoid species (mainly the genera Cystoseira) reach deeper waters in comparison with corticated foliose algae, at fished sites.

Drought refuges affect algal recolonization in intermittent streams

AbstractDrought refuges enable the biota of intermittent streams to survive dry periods, but their roles in sustaining algal assemblage structure and productivity are not known. This study determined the importance of two types of drought refuge (dry biofilm, permanent pools) for benthic algal regrowth in four intermittent streams in the Grampians National Park, Victoria, Australia. Dry biofilm was removed by scrubbing randomly chosen stones from two dry riffles in each of two streams with permanent pools and two streams without permanent pools. Algae were sampled from scrubbed and control stones at one, six and sixteen weeks after flow resumed. Removal of dry algal biofilm resulted in lower algal densities on stones from both stream types, and scrubbed stones had similar densities of algae after one week, regardless of the presence or absence of permanent pools. Algal density on stones exposed to both refuge types was more than an order of magnitude higher than the density on stones exposed to either one or neither refuge. The effects of biofilm removal on algal density were still apparent after six weeks of flow and assemblage composition differed between streams with and without permanent pools. Dry biofilm was an important drought refuge for algae in both stream types, but epilithic algal density in streams with permanent pools developed more rapidly than streams that were completely dry over summer. Loss of permanent pools from intermittent streams due to water abstraction may substantially reduce algal productivity after flow resumes, which may limit the supply of autochthonous carbon. Copyright © 2004 John Wiley &amp; Sons, Ltd.

The effects of light and thallus scour from Ecklonia radiata canopy on an associated foliose algal assemblage: the importance of photoacclimation

Kelp canopies have long been recognised for their influence on the structure of algal assemblages on sublittoral reefs. In Marmion Lagoon, Western Australia, we investigated how a canopy of the small kelp Ecklonia radiata affected the associated foliose algal assemblage and what potential mechanisms were responsible for the effects we observed. Light levels and physical abrasion (thallus scour) by an E. radiata canopy were manipulated in an orthogonal fixed-factor experiment that revealed that changes in light, but not thallus scour, had a significant effect on the composition of the algal assemblage. Reduced light levels were associated with a decrease in the Shannon-Wiener diversity index and an increased dominance of the foliose algae Pterocladia lucida and Rhodymenia sonderi. Photobiological investigations of three foliose species, P. lucida, R. sonderi and Chauviniella coriifolia indicated that they were able to photoacclimate to low light levels by increasing thallus absorption by up to 11%. Photoacclimation was also evident by an increase in the maximum rate of electron transport under low-light conditions. We conclude that the E. radiata canopy in Marmion Lagoon structures the foliose algal assemblage through the modification of the light environment and that this effect may be mediated by differences in the ability of different species of foliose algae to photoacclimate.

NUTRIENT ENHANCEMENT INCREASES PERFORMANCE OF A MARINE HERBIVORE VIA QUALITY OF ITS FOOD ALGA

Herbivore performance and reproductive output are expected to covary with qualitative changes in the host plant. The availability of nutrients may be especially important for the quality of algae as food through its effect on both primary and secondary metabolism and within-plant variation. We evaluated the effect of the nutrient enrichment of the brown alga Fucus vesiculosus on the fecundity, mass gain, intermolt duration, and food consumption of females of the herbivorous isopod Idotea baltica. We reared the isopods on algae grown either under enhanced nutrient conditions or as controls for the entire duration of the intermolt preceding mating and oviposition. The herbivores fed on nutrient-treated algae achieved higher growth rates, consumed more food, and laid more and larger eggs than the controls. The nutrient treatment increased the amount of insoluble sugars and reduced both the total carbon content in the alga and the physical toughness of the thallus. On the other hand, the treatment had no effect on the amount of total nitrogen or phlorotannins in the algal tissue. This implies that the clear increase in herbivore performance and fecundity may be due to quantitative changes in the carbohydrate metabolism rather than to the lower content of feeding deterrents in the tissue of F. vesiculosus. We suggest that eutrophication may potentially improve the quality of F. vesiculosus for herbivores. The reproductive success, growth, and food consumption of I. baltica increase on algae maintained in a nutrient-rich environment; this is likely to be reflected in herbivore density and grazing pressure. This in turn may have potential ecological consequences for the maintenance of viable F. vesiculosus belts, thereby contributing to the structure of littoral algal assemblages in marine ecosystems.

Opposing states of subtidal habitat across temperate Australia: consistency and predictability in kelp canopy-benthic associations

We tested the view that few generalizations are possible about the structure of subtidal algae assemblages and that the situation worsens or does not change as more sites are examined. We quantified the percentage cover of 4 morphological groups of benthic algae (encrusting and articu- lated coralline algae, and foliose and turfing algae) under canopies of kelp Ecklonia radiata and in areas without kelp (macroalgae >4 cm high). This was repeated over 4 spatial scales ranging from quadrats (separated by 10s of m), sites (separated by km), locations (separated by 100s of km) to regions (separated by 1000s of km) across the southern coastline of Australia: Western Australia (WA), Southern Australia (SA) and Eastern Australia (EA). The key result was that while comparison among sites revealed substantial and often inconsistent variation in abundance of benthic algae between habitats (kelp vs open), clear patterns emerged when locations and regions were compared. In EA, where grazers are effective in maintaining extensive areas of encrusting corallines, patterns of algal cover between habitats (kelp vs open) were generally reversed to WA and SA, where grazers are sub- stantially less effective. These results indicate a large distinction in the ecology of these regions (WA = SA ≠ EA) and how lack of understanding of this pattern at the regional scale tends to suggest overwhelming variation when single studies are compared among regions. These differences also highlight that comparisons of studies done at small scales, even if done at several sites in a locality, provide a difficult basis to understand the generality of pattern in algal assemblage structure due to large variation at this scale. While we acknowledge that these broad patterns were not possible to validate at the scale of sites, it was possible to increase the scale of observation to encompass broader patterns that might be organized around a relatively simple set of ecological predictions.

Effects of sea urchin (Evechinus chloroticus) grazing in Dusky Sound, Fiordland, New Zealand

Evechinus chloroticus (Val.) is known to be an important grazer of subtidal barren habitats in northern New Zealand but its role in structuring rocky reef communities at colder southern localities is poorly understood. The present study was prompted by a proposal for the establishment of an E. chloroticus fishery in Dusky Sound, Fiordland, New Zealand. To determine the ecological importance of E. chloroticus in Dusky Sound, sea urchins were experimentally removed from shallow fucoid fringe (upper zone), mid‐depth barrens (middle zone), and deeper algal meadow (lower zone), at eight sites and compared with a paired set of eight control sites. Two years of continuous removal of E. chloroticus resulted in conspicuous changes to the algal community at all depth zones examined. The density of fucoid algae (Carpophyllum spp., Sargassum spp., Cystophora spp., and Landsburgia quercifolia) increased mostly in the upper and middle zones, although smaller increases were also observed in the lower zone. Changes in cover of herbaceous, turfing and crustose algae observed in the middle and lower zones were larger than in the upper zone. Density increases of the kelp Ecklonia radiata were observed mainly in the lower zone, although still important in the middle zone. These results show that E. chloroticus can have a strong influence on the structure of algal assemblages in Dusky Sound hence the development of an E. chloroticus fishery in this area should be treated with caution.

Ingestion and digestion of epilithic algae by larval insects in a heavily grazed montane stream

Summary1. Epilithic algae grown on elevated or non‐elevated ceramic tiles were exposed (to produce assemblages with different grazing histories) in a heavily grazed, montane stream in New Mexico, U.S.A. to Ameletus nymphs (Ephemeroptera) and Ecclisomyia larvae (Trichoptera) and the algal composition in insect faeces was compared to that on the tiles. Differences in grazing and digestion efficiency between grazers were then assessed and also differences in susceptibility to ingestion and digestibility among common algae.2. Ordination of tile and faecal samples, using the relative abundance of common algae, revealed that: (i) algal assemblages on elevated vs. non‐elevated tiles differed only slightly; (ii) the taxonomic composition of algae in faeces of both caddis and mayflies differed substantially from that on the tiles, indicating low grazing efficiency for some algal taxa; and (iii) the algal composition of faeces produced by caddis larvae and mayflies was similar, indicating little difference in grazing efficiency between them. However, some algal taxa were more susceptible to ingestion by caddisfly larvae when occurring on elevated tiles than on non‐elevated tiles, suggesting that previous exposure to caddis grazing influenced assemblage attributes.3. Although Ameletus and Ecclisomyia differed little in grazing efficiency, the percentage of diatoms that were dead after passage through the gut was greatest in the mayfly treatment, suggesting that mayflies digested diatoms more efficiently than the caddis. Analyses of differences in the condition of chloroplasts within diatoms in tile and faecal samples showed that losses of ’live‘ diatom cells (i.e. those containing full chloroplasts) during gut passage through mayflies equalled the increase, in faeces, of ’dead‘ (empty frustules) cells of all common diatoms. In contrast, some diatoms were digested inefficiently by caddis larvae.4. Algae on elevated tiles contained a higher proportion of dead diatoms than those on non‐elevated tiles, possibly because mayflies visited raised tiles more often and, consequently, ingested and defaecated cells at a higher rate in the absence of caddis larvae. Moreover, diatom taxa differed in the percentage of cells that were dead within tile assemblages, with populations of typically grazer‐resistant taxa (e.g. Achnanthidium minutissimum, Planothidium lanceolatum and Cocconeis placentula var. euglypta) containing significantly more dead cells than grazer‐susceptible taxa [e.g. small, chain‐forming Fragilaria (= Staurosirella)]. This result suggests that a trade‐off exists between ingestion vs. digestion resistance of microalgae. Both the ingestion and digestion efficiency of algivorous macroinvertebrates could influence the structure and function of algal assemblages. In heavily grazed systems, where algal cells are probably processed through grazer guts repeatedly, differential resistance to digestion among algae may be particularly important.

Fishing, Trophic Cascades, and the Structure of Algal Assemblages: Evaluation of an Old but Untested Paradigm

Removal of important predators by fishing can result in trophic cascades and indirect effects on marine benthic communities. Indirect effects are especially evident when prey populations released from predation by fishing have the ability to modify entire benthic communities as do sea urchins Sea urchins have been shown to dramatically alter the underwater landscape by grazing, by converting stands of large erect algae into coralline barrens. In the western Mediterranean, a recent extension of coralline barrens into areas formerly dominated by erect algal assemblages has been attributed to release of predation on sea urchins by overfishing. Most suggestions concerning the transition form erect algeal assemblages to coralline barrens, however, have been speculative, and little descriptive and experimental work has been carried out to verify the hypothesis that fish predation on sea urchins (and its subsequent release by overfishing) drives this transition. Here we critically review the literature concerning the effect of fishing on sea urchin populations and its subsequent maintenance of different algal assemblages in the Mediterranean. The extant data cannot refute the fishes as important predators model. but we argue that other processes trecruitment, pollution, disease, large-scale oceanographic events, sea urchin harvesting, food subsidies, and availability of shelters) may also be important in regulating the structure of Mediterranean algal assemblages.

Low densities of sea urchins influence the structure of algal assemblages in the western Mediterranean

Numerous studies of interactions between urchins and algae in temperate areas have shown an important structuring effect of sea urchin populations. These studies focused almost wholly on the effect of high urchin densities on laminarian forests. In contrast, algal communities below 5–6 m depth in the northwestern Mediterranean are characterised by low sea urchin densities (<5 ind m −2) and the absence of laminarian forests. No previous research has addressed sea urchin/algal interactions in this type of community. To determine the effect of the most abundant echinoid species, Paracentrotus lividus, on well-established algal communities in this area, we performed a removal–reintroduction experiment in rocky patches located between 13 and 16 m depth in the northwestern Mediterranean, where sea urchin densities ranged between 0.9 and 3.4 ind m −2. After 6 months, the cover of non-crustose algae was significantly higher in the plots from which sea urchins had been removed than in control plots (84 vs 67% cover). These removal plots reverted to their original state upon reintroduction of sea urchins. The non-crustose algae consisted of turfing and frondose forms, with the former representing some 70% of the non-crustose algal cover. Change in the cover of turfing algae was responsible for the significant increase in algal development in the sea urchin removal plots. The response of frondose algae to the treatment varied between algal species. It is concluded that grazing by P. lividus exerts a significant effect on habitat structure, even in communities with low sea urchin densities, such as those found in vast areas of the Mediterranean sublittoral.