Increased macroalgal abundance following mass mortalities of sea urchins (Strongylocentrotus droebachiensis) along the Atlantic coast of Nova Scotia

Summary Recurrent outbreaks of disease between 1980 and 1983 caused catastrophic mortality of sea urchins (>260,000 t fresh weight) along 280 km (straight line distance) of the Atlantic coast of Nova Scotia. The complete elimination of sea urchins and concomitant development of fleshy macroalgal communities have occurred along different parts of this coast in different years. Macroalgal communities in areas where sea urchins died off 1, 3 and 4 years previously are compared to existing sea urchin-dominated barren grounds and to a mature kelp bed without sea urchins. Changes in macroalgal cover and species composition, and increases in biomass, density and size of kelp (Laminaria) species, characterize the succession from barren grounds to 3- and 4-year-old kelp beds. The greatest change occurred between one and three years following sea urchin mass mortality. Within 3 years, kelp beds attained a level of biomass (7.6 kg m^-2) comparable to that of mature beds. Recovery of sea urchin populations via recruitment of planktonic larvae has been slow and spatially variable. Large-scale reciprocal fluctuations in kelp and sea urchin biomass may characterize the trajectory of a dynamic system which cycles between two alternate community states: kelp beds and sea urchin-dominated barren grounds. Periodic decimation of sea urchin populations by disease may be an important mechanism underlying this cyclicity.     

The influence of herbivores on primary producers can vary spatially and interact with disturbance

Identifying the major drivers of ecosystem change remains a central area of ecological research. Although top–down drivers of change have received particular focus, we still have little understanding of how consistently these factors control an ecosystem's shift in both directions, between different ecosystem states. Using a crossed experiment in a shallow embayment in southeastern Australia, we investigated the roles of disturbance (kelp removal) and sea urchin herbivory (via increased density) to determine their contributions to shifts away from a kelp‐dominated community. In a second experiment, done in urchin barren areas at two sites, we tested whether reductions in ambient sea urchin densities allowed an algal shift in the reverse direction. In both experiments, we observed that high densities of sea urchins could negatively influence kelp and macroalgal abundance. However, in the kelp bed, a moderate or severe disturbance resulted in a comparable algal response, irrespective of urchin density. The influence of sea urchins also varied dramatically between the two urchin barren sites. Here, reducing urchin densities resulted in algal recovery at one site, but at the other site, substantial colonisation of barren areas by canopy‐forming brown algae and Ulvales occurred across all (low, medium, and high) urchin density treatments. Our findings illustrate multiple pathways of urchin barren creation and algal recovery, and reveal that shifts both to and from an urchin barren state can occur irrespective of herbivore pressure. These alternate pathways can operate over short spatial distances or with different regimes of disturbance.     

Can multitrophic interactions and ocean warming influence large‐scale kelp recovery?

Ongoing changes along the northeastern Atlantic coastline provide an opportunity to explore the influence of climate change and multitrophic interactions on the recovery of kelp. Here, vast areas of sea urchin‐dominated barren grounds have shifted back to kelp forests, in parallel with changes in sea temperature and predator abundances. We have compiled data from studies covering more than 1,500‐km coastline in northern Norway. The dataset has been used to identify regional patterns in kelp recovery and sea urchin recruitment, and to relate these to abiotic and biotic factors, including structurally complex substrates functioning as refuge for sea urchins. The study area covers a latitudinal gradient of temperature and different levels of predator pressure from the edible crab (Cancer pagurus) and the red king crab (Paralithodes camtschaticus). The population development of these two sea urchin predators and a possible predator on crabs, the coastal cod (Gadus morhua), were analyzed. In the southernmost and warmest region, kelp forests recovery and sea urchin recruitment are mainly low, although sea urchins might also be locally abundant. Further north, sea urchin barrens still dominate, and juvenile sea urchin densities are high. In the northernmost and cold region, kelp forests are recovering, despite high recruitment and densities of sea urchins. Here, sea urchins were found only in refuge habitats, whereas kelp recovery occurred mainly on open bedrock. The ocean warming, the increase in the abundance of edible crab in the south, and the increase in invasive red king crab in the north may explain the observed changes in kelp recovery and sea urchin distribution. The expansion of both crab species coincided with a population decline in the top‐predator coastal cod. The role of key species (sea urchins, kelp, cod, and crabs) and processes involved in structuring the community are hypothesized in a conceptual model, and the knowledge behind the suggested links and interactions is explored.     

Effects of location,exposure and physical structure on juvenile recruitment of the sea urchin Strongylocentrotus droebachiensis in the Gulf of Maine

Summary

Studies of juvenile recruitment of the green sea urchin Strongylocentrotus droebachiensis in the Gulf of Maine were conducted during the summer of 1995. These experiments confirmed 12 years of previous observations that settlement only occurs during the months of June and July. Settlement panels were placed at a series of sites along the Maine and New Hampshire coastline to compare recruitment in the northeastern and southwestern regions of the Gulf of Maine. The densities of urchins recruiting in Casco Bay and at the Isles of Shoals were two orders of magnitude higher than those from Eastport and Winter Harbor. There was a discontinuity in settlement densities at Penobscot Bay. Experiments conducted at the Isles of Shoals showed a positive relationship between water motion and larval supply, but neither parameter correlated with recruitment density over eight stations. Contrary to previous results, recuitment was greater within natural, as well as, artificial kelp beds compared to urchin barren areas and control panels outside the experimental kelp beds. The impact of changing community structure due to urchin harvesting was discussed as an factor influencing differences in juvenile urchin recruitment.     

Seasonal changes in subarctic sea urchin populations from different habitats

This paper documents seasonal variation in certain sea urchin (Strongylocentrotus polyacanthus) characteristics in habitats of varying environmental conditions. At Shemya Island, Alaska, three habitat types [dense kelp beds, intermediate kelp beds, and algal barrens (low to no foliose algal cover)] were monitored seasonally from September 1995 to August 1996, for live and drift foliose algae. In general, drift algal abundance was greater in areas with more attached kelp, but this varied with season. Along with drift algae, sea urchin density, test size diameter, gonad and nutrition indices, and mobility were seasonally sampled within each habitat. Densities were highest in the algal barrens and lowest in the kelp beds. Seasonally, densities varied between summer/fall, and winter/spring, with lower numbers in the winter/spring. Test size was largest in the kelp habitats when compared to the intermediate or barren sites. Test size was seasonally consistent in the kelp habitats but not in the intermediate or barren sites. Here, test size did vary depending on season (larger urchins were found in winter). The gonad index showed much seasonal variation at the kelp and intermediate kelp sites, but was relatively more stable over time in the barren habitats. Between habitats, gonad and nutrition indices were larger in areas with kelp. Urchin movement varied seasonally between habitats, with more overall movement and variation in barren habitats. These results illustrate the importance of small-scale temporal and spatial variation. Monitoring for 1 year demonstrated that certain parameters varied more in areas of higher foliose algal cover (gonad indices), while other parameters varied more in low kelp areas (test size and movement). These results suggest that studies involving urchins should consider both time of year and overall algal community composition when conducting any type of experimental or monitoring work.     

Biological interactions maintain the boundaries between kelp forests and urchin barrens in the Aleutian Archipelago

This study used benthic surveys and manipulative experiments to examine (1) if boundaries between kelp forests and urchin barrens exist at multiple locations spanning the Aleutian Archipelago, (2) if these boundaries are spatially stable, and (3) how changes in algal density within the kelp forests influence the ability of urchins to invade them. Our results demonstrate that sharply punctuated kelp forest-urchin barren boundaries occur throughout the Archipelago, and they are spatially stable for at least 2 years. Further, when all macroalgae were experimentally removed from the kelp forest side of the boundaries, urchins rapidly moved into these clearings and excluded macroalgae for up to 2 years. However, these movements were not observed where 75% or less of the macroalgae was removed (leaving 25% or more in place), suggesting that even low macroalgal abundances can prevent urchins from invading the kelp forests. Further, urchin densities were negatively related to kelp density, again indicating that kelp can reduce urchin densities. While the ability of urchins to overgraze kelp forests is widely known, our results indicate that kelp can inhibit urchins, that these inhibitory influences are a widely recurrent phenomenon, and that this interaction is important to maintaining kelp forests across the Aleutians.     

The efficiency and effectiveness of different sea urchin removal methods for kelp forest restoration

Sea urchin overgrazing has caused widespread phase shifts from kelp forests to “urchin barrens” on many temperate reefs, reducing habitat complexity, productivity, and biodiversity. Sea urchin removal is increasingly used for kelp restoration; however, few studies have quantified the efficiency and effectiveness of different removal methods, resulting in limited understanding of their practicality. In this study, the efficiency (removal rate) and effectiveness (proportion removed) of four removal methods were evaluated in northeastern New Zealand. We compared culling or collecting sea urchins by either SCUBA or freediving in 128 small-scale plots (25 m²). We also evaluated the efficiency and effectiveness of culling in four large (1.6–2 ha) barren areas, scales relevant for restoration. On average, culling sea urchins was 1.9–4.4 times faster than collecting, and SCUBA was 1.5–3.3 times faster than freediving. Removal rates increased with sea urchin density, especially for culling on SCUBA, while freediving removal rates increased with experience. Effectiveness was lower in large-scale removals (86–93% of sea urchins ≥40 mm removed) compared to small-scale removals (98–99%), but sufficient for restoration objectives. Estimated time per area (using SCUBA culling) was similar across large-scale removals (49–57 hours/ha), despite an almost 2-fold variation in initial sea urchin densities (approximately 4–8 urchins/m²), suggesting area may better predict total removal time than simply number of sea urchins across low-density ranges. While sea urchin removal provides a rapid, feasible, and effective approach to restoring kelp in urchin barrens, restoration plans need to also address the causes of sea urchin overpopulation to ensure long-term benefits.     

Overgrazing of Kelp Beds Along the Coast of Norway

The aim of this study was to better understand the down-grazing of kelp beds by sea urchins (Strongylocentrotus droebachiensis) along the coast of Norway. Barren grounds were first observed in sheltered areas along the coast of the counties of Trø ndelag, Nordland and Troms in 1974. In the 1980s, the barren grounds spread to areas more heavily exposed to waves. In the 1990s, the kelp beds were re-established in some localities in southern Trø ndelag, initially in wave-exposed areas. In the northernmost parts of Norway, i.e. the counties of Troms and Finnmark, the barren ground areas may still be increasing. Crabs (Cancer pagurus) and common eiders are the most common predators on urchins. Predation on sea urchins in kelp beds is probably not among the factors that limit the sea urchin populations. Along the coast of Nordland and further north, sea urchins are infected by nematodes, resulting in a low, but significant increase in their mortality. No re-growth of kelp beds has been found in the most infected areas. In the late 1960s and the early 1970s, a high occurrence of echinoderm larvae was observed in deeper waters. This was a period with cold water, which may have caused high recruitment of sea urchins. The bet-hedging life strategy of sea urchins may account for the sudden increase in the size of the populations. In the present paper I propose the hypothesis that higher individual growth rates and higher mortality rates in the south than in the north may explain the decrease in the populations, which may in turn account for the re-growth of kelp in the southern areas.     

Grazing intensity influences the strength of an associational refuge on temperate reefs

Recent studies have emphasized the role of positive interactions in ecological communities, but few have addressed how positive interactions are mediated by abiotic stress and biotic interactions. Here, I investigate the effect of a facilitator species on the abundance of macroalgae over a gradient of herbivory. Grazing by sea urchins can be intense on temperate reefs along the California coast, with benthic macroalgae growing exclusively in physical refuges and interspersed within colonies of the strawberry anemone, Corynactis californica. Field experiments indicated that the net effect of C. californica on turf algae was strongly nonlinear over a gradient in density of sea urchins. At low intensities of urchin grazing, the anemone and macroalgae competed for space, with algae capable of overgrowing C. californica. At intermediate grazing intensities, C. californica provided a refuge for turf algae but not for juvenile kelp. Neither turf algae nor kelp benefited from the presence of C. californica at the highest levels of grazing intensity, as sea urchins consumed nearly all macroalgae. The hump-shaped effect observed for C. californica contrasts with the prevailing view in ecological theory that positive interactions are more common in harsh environmental conditions. The results reported here qualify this view and underscore the need to evaluate positive interactions over a range of abiotic stress and consumer pressure.     

Urchin grazing of kelp gametophytes in warming oceans

Sea urchins can cause extensive damage to kelp forests, and their overgrazing can create extensive barren areas, leading to a loss of biodiversity. Barrens may persist when the recruitment of kelp, which occurs through the microscopic haploid gametophyte stage, is suppressed. However, the ecology of kelp gametophytes is poorly understood, and here we investigate if grazing by juvenile urchins on kelp gametophytes can suppress kelp recruitment and if this is exacerbated by climate change. We compared grazing of Ecklonia radiata gametophytes by two species of juvenile urchins, the tropical Tripneustes gratilla and the temperate Centrostephanus rodgersii, at winter (19°C), summer (23°C), and ocean warming (26°C) temperatures for the low-latitude range edge of E. radiata, which is vulnerable to ocean warming. We examined the rate of recovery of gametophytes following grazing and determined whether they survived and formed sporophytes after ingestion by sea urchins. Both T. gratilla and C. rodgersii grazed E. radiata gametophytes, reducing their abundance compared to no grazing controls. Surprisingly, temperature did not influence grazing rates, but gametophytes did not recover from grazing in the ocean warming (26°C) treatment. Gametophytes survived ingestion by both species of sea urchin and formed sporophytes after ingestion by T. gratilla, but not C. rodgersii. These results suggest complex grazer–gametophyte interactions, in which both negative (reduced abundance and poor recovery with warming) and positive (facilitated recruitment) effects are possible. Small grazers may play a more important role in kelp ecosystem function than previously thought and should be considered in our understanding of alternate stable states.     

Utilization of purple and red sea urchins (Strongylocentrotus purpuratus Stimpson and S. franciscanus Agassiz) as food by the white sea urchin (Lytechinus anamesus Clark) in the field and laboratory

White sea urchins (Lytechinus anamesus Clark) attacked purple (Strongylocentrotus purpuratus Stimpson) and red (S. franciscanus Agassiz) sea urchins at Anacapa Island, California. Densities of white urchins were highest in the deep algal crust-dominated community where up to 6% of purple and 25% of red urchins were being attacked by white urchins. Up to 9% of Lytechinus anamesus in an area were actively eating stronglylocentrotids and usually, more than one white urchin was involved in the attack. In areas with low densities of white urchins, no strongylocentrotids were being attacked.After 36 h in the laboratory, there was no difference in the number of white urchins attacking injured or healthy purple urchins in each of the three experimental densities of white urchins. However, both injured and healthy urchins were attacked by more white urchins in high density. When given a choice between injured purple urchins or fresh kelp, white urchins overwhelmingly chose kelp. Data suggest that white urchins utilize other urchin species as an alternative source of food when more preferred food is absent, but will switch to preferred food should it become available.     

Destruction of kelp-beds by sea-urchins: A cyclical phenomenon or irreversible degradation?

A stable kelp bed ecosystem in St. Margaret's Bay, Nova Scotia (Canada), had as its main producersLaminaria longicruris andL. digita. Most algal production was exported as detritus, but there was a moderate population of herbivores, mainly the sea urchinsStrongylocentrotus droebachiensis. These were eaten by crabs,Cancer irroratus and by lobsters,Homarus americanus. Lobsters also preyed on crabs. Beginning in 1968, sea urchins became locally abundant and overgrazed the kelp beds, converting large areas to urchindominated barren grounds. Almost all kelp beds in St. Margaret's Bay (140 km²) have now been destroyed. During the same period, lobster biomass decreased, and the hypothesis was put forward that reduction in lobster predation led to increased urchin abundance and kelp bed destruction. Evidence is presented for the hypothesis that urchin-dominated barren grounds are a new, stable configuration of the ecosystem, and that a long-term decrease in primary and secondary productivity of these coastal waters can be expected.     

Trophic Cascades Induced by Lobster Fishing Are Not Ubiquitous in Southern California Kelp Forests

Fishing can trigger trophic cascades that alter community structure and dynamics and thus modify ecosystem attributes. We combined ecological data of sea urchin and macroalgal abundance with fishery data of spiny lobster (Panulirus interruptus) landings to evaluate whether: (1) patterns in the abundance and biomass among lobster (predator), sea urchins (grazer), and macroalgae (primary producer) in giant kelp forest communities indicated the presence of top-down control on urchins and macroalgae, and (2) lobster fishing triggers a trophic cascade leading to increased sea urchin densities and decreased macroalgal biomass. Eight years of data from eight rocky subtidal reefs known to support giant kelp forests near Santa Barbara, CA, USA, were analyzed in three-tiered least-squares regression models to evaluate the relationships between: (1) lobster abundance and sea urchin density, and (2) sea urchin density and macroalgal biomass. The models included reef physical structure and water depth. Results revealed a trend towards decreasing urchin density with increasing lobster abundance but little evidence that urchins control the biomass of macroalgae. Urchin density was highly correlated with habitat structure, although not water depth. To evaluate whether fishing triggered a trophic cascade we pooled data across all treatments to examine the extent to which sea urchin density and macroalgal biomass were related to the intensity of lobster fishing (as indicated by the density of traps pulled). We found that, with one exception, sea urchins remained more abundant at heavily fished sites, supporting the idea that fishing for lobsters releases top-down control on urchin grazers. Macroalgal biomass, however, was positively correlated with lobster fishing intensity, which contradicts the trophic cascade model. Collectively, our results suggest that factors other than urchin grazing play a major role in controlling macroalgal biomass in southern California kelp forests, and that lobster fishing does not always catalyze a top-down trophic cascade.     

Latitudinal variation in intertidal algal community structure: the influence of grazing and vegetative propagation

Summary The hypothesis that sea urchin grazing and interactions with turf-forming red algae prevent large brown algae from forming an extensive canopy in the low intertidal zone of southern California was tested with field experiments at two study sites. Experimental removal of sea urchins resulted in rapid algal recruitment. Crustose coralline algae which typically dominate the substratum in areas with dense urchin populations were quickly overgrown by several species of short-lived green, brown and red algae. The removal of urchins also significantly increased the recruitment of two long-lived species of large brown algae (Egregia laevigata and Cystoseira osmundacea at one study site and E. laevigata and Halidrys dioica at the other). The experimental plots at both sites were eventually dominated by perennial red algae.A two-factorial experiment demonstrated that sea urchin grazing and preemption of space by red algae in areas where urchins are less abundant are responsible for the rarity of large brown algae in the low intertidal of southern California. The three dominant perennial red algae, Gigartina canaliculata, Laurencia pacifica and Gastroclonium coulteri, recruit seasonally from settled spores but can rapidly fill open space with vigorous vegetative growth throughout the year. These species encroach laterally into space created by the deaths of large brown algae or by other disturbances. Once extensive turfs of these red algae are established further invasion is inhibited. This interaction of algae which proliferate vegetatively with algae which recruit only from settled spores is analogous to those which occur between solitary and colonial marine invertebrates and between solitary and cloning terrestrial plants.It is suggested that a north-south gradient in the abundance of vegetatively propagating species, in grazing intensity and in the frequency of space-clearing disturbances, may account for latitudinal variation in intertidal algal community structure along the Pacific coast of North America.     

Do native herbivores provide resistance to Mediterranean marine bioinvasions? A seaweed example

Generalist herbivores in marine ecosystems are poorly examined for their potential to serve as a source of biotic resistance against algal invasion. We assessed how one of the main generalist herbivores in Mediterranean rocky reefs (the sea urchin Paracentrotus lividus) affects Lophocladia lallemandii and Caulerpa racemosa, two algal invaders with strong detrimental effects on native benthic communities. In a comparison of sea urchin gut contents to algal community composition, strong preferences were exhibited, leading to no relationship between consumption and availability. Both C. racemosa and L. lallemandi were abundant in algal assemblages (>60% occurrence), but C. racemosa (20% of diet) was consumed more than L. lallemandi (3.5%). Experimental enclosures of sea urchins (12 sea urchins * m⁻²) were carried out in locations where L. lallemandii was already established and C. racemosa was rare (new invasion) or abundant (established invasion). C. racemosa was negatively affected by sea urchins only when it was rare, and no effect was detected when the alga was already abundant. Results for L. lallemandi were exactly opposite: urchins limited seasonal increases in L. lallemandi in highly-invaded areas. Because of the small amount of direct consumption of L. lallemandi, its decrease in abundance may be related to the grazing of native algae where L. lallemandii is attached. Overall, our results show that high densities of native herbivores may reduce invasive algae at low densities, due to a combination of direct and indirect effects, but it has no significant effect in highly-invaded areas.     

Long Term Variability in the Structure of Kelp Communities in Northern Chile and the 1997–98 ENSO

This is the first study on the south eastern Pacific coast of South America which details long term, interannual variability in the structure of subtidal rocky-bottom kelp-dominated communities before, during, and after the El Niño Southern Oscillation (ENSO) event of 1997–1998 in northern Chile (23^°S). The temporal patterns of the main components of these ecosystems, which included Macrocystis integrifolia, Lessonia trabeculata, echinoids and asteroids, were evaluated seasonally between 1996 and 2004. M. integrifolia demonstrated high interannual variability in temporal patterns of abundance. The 1997–1998 ENSO did not significantly modify the temporal patterns of Macrocystis, although local extinction of M. integrifolia beds occurred during negative thermal anomalies in 1999–2000 (La Niña event), facilitating the establishment of urchin dominated “barren grounds”. The abundance of Lessonia trabeculata showed little temporal variability, and this species dominated the deeper regions of the kelp assemblage (8–13 m depth). The structure of the kelp communities in the study area is regulated by a trophic cascade which modulates alternation between kelp dominated areas and sea urchin barrens. In this context, frequent and intense upwelling of cold water high in nutrients favors the establishment and persistence of kelp assemblages. During ENSO, coastal upwellings can mitigate superficial warming of coastal water and increase the nutrient concentration in the water column. Superficial warming during the 1997–1998 ENSO induced spawning by different species of echinoderms, which resulted in major recruitment of these species during 1999. Top-down events, such as the decrease in densities of the asteroids after the 1997–1998 ENSO event, favored increases in densities of benthic grazers, which caused significant decreases in abundance of M. integrifolia. The re-establishment of the adult fraction of the carnivore (starfish) guild coincided with a decrease in the density of sea urchins and thus re-establishment of the kelp. In the temperate south eastern Pacific, oceanographic events, which act on different spatial-temporal scales, trigger trophic cascades that act at local levels, producing interannual variability in the structure of kelp communities. On the other hand, considering the high macroinvertebrate diversity associated with kelp assemblages, the transitions between kelp-dominated areas and sea urchin barrens do not appear to significantly affect the biodiversity of these assemblages of benthic invertebrates.     

Do sulphuric acid and the brown alga <Emphasis Type="Italic">Desmarestia viridis</Emphasis> support community structure in Arctic kelp patches by altering grazing impact,distribution patterns,and behaviour of sea urchins?

Macrobenthic community structure and the distribution of the green sea urchin (Strongylocentrotus droebachiensis) were recorded inside and outside (=barrens) of kelp patches (Alaria esculenta) at Kongsfjordneset, Svalbard between August 2002 and October 2006. In manipulative field experiments, conducted at Kongsfjordneset, Svalbard in August 2002, the effect of the presence of the brown seaweed Desmarestia viridis on sea urchin distribution and kelp grazing was determined. Additionally, we studied the effect of sulphuric acid, which is produced and stored by D. viridis, on sea urchin movements in the laboratory at Ny-Ålesund, Svalbard, in May 2006. Sea urchin densities were two- to threefold lower in kelp patches than on barrens. The macrobenthic community inside kelp patches hosted 39% more species and was of different species composition than on barrens. Anchored pieces of the kelp A. esculenta were less consumed when surrounded by D. viridis than non-surrounded conspecifics. Changes in pH affected the behaviour of sea urchins. Exposing sea urchins to 500 μl seawater at pH 7.5 caused them to stop, while the exposure of as little as 25 μl at pH 1 caused sea urchins to move in the opposite direction. Acid-mediated escape responses in sea urchin behaviour suggest chemical protection by D. viridis as an additional mechanism to mechanical protection in the generation of kelp refuges. These results improve our understanding of how isolated kelp beds can persist over a wide range of environmental conditions, like wave-sheltered sites, and suggest that changes in community structure may be in part attributable to altered trophic interactions.     

Testing the generality of the trophic cascade paradigm for sea otters: a case study with kelp forests in northern Washington,USA

Trophic cascade hypotheses for biological communities, linking predation by upper trophic levels to major features of ecological structure and dynamics at lower trophic levels, are widely subscribed and may influence conservation policy. Few such hypotheses have been evaluated for temporal or spatial generality. Previous studies of sea otter (Enhydra lutris) predation along the outer coast of North America suggest a pattern, often elevated to the status of paradigm, in which sea otter presence leads to reduced sea urchin (Strongylocentrotus spp.) biomass and rapid increases in abundance and diversity of annual algal species, followed by a decline in diversity as one or a few perennial algal species become dominant. Both sea otter predation and commercial sea urchin harvest are ecologically and economically important sources of urchin mortality in nearshore benthic systems in northern Washington marine waters. We recorded changes in density of macroalgae in San Juan Channel, a marine reserve in the physically protected inland waters of northern Washington, resulting from three levels of experimental urchin harvest: (1) simulated sea otter predation (monthly complete harvest of sea urchins), (2) simulated commercial urchin harvest (annual size-selective harvest of sea urchins), and (3) no harvest (control). The two experimental urchin removal treatments did not significantly increase the density of perennial (Agarum and Laminaria) or annual (Desmarestia, Costaria, Alaria and Nereocystis) species of macroalgae after 2 years, despite significant and persistent decreases in urchin densities. Our results suggest that other factors such as grazing by other invertebrates, the presence of dense Agarum stands, and recruitment frequency of macroalgae and macroinvertebrates may play a large role in influencing community structure in San Juan Channel and other physically protected marine waters within the range of sea otters. Handling editor: J. Trexler     

Kelp and sea urchin settlement mediated by biotic interactions with benthic coralline algal species

Species interactions can influence key ecological processes that support community assembly and composition. For example, coralline algae encompass extensive diversity and may play a major role in regime shifts from kelp forests to urchin-dominated barrens through their role in inducing invertebrate larval metamorphosis and influencing kelp spore settlement. In a series of laboratory experiments, we tested the hypothesis that different coralline communities facilitate the maintenance of either ecosystem state by either promoting or inhibiting early recruitment of kelps or urchins. Coralline algae significantly increased red urchin metamorphosis compared with a control, while they had varying effects on kelp settlement. Urchin metamorphosis and density of juvenile canopy kelps did not differ significantly across coralline species abundant in both kelp forests and urchin barrens, suggesting that recruitment of urchin and canopy kelps does not depend on specific corallines. Non-calcified fleshy red algal crusts promoted the highest mean urchin metamorphosis percentage and showed some of the lowest canopy kelp settlement. In contrast, settlement of one subcanopy kelp species was reduced on crustose corallines, but elevated on articulated corallines, suggesting that articulated corallines, typically absent in urchin barrens, may need to recover before this subcanopy kelp could return. Coralline species differed in surface bacterial microbiome composition; however, urchin metamorphosis was not significantly different when microbiomes were removed with antibiotics. Our results clarify the role played by coralline algal species in kelp forest community assembly and could have important implications for kelp forest recovery.     

Changing Community States in the Gulf of Maine: Synergism Between Invaders, Overfishing and Climate Change

Human activities, including overfishing and species introductions, have had a dramatic impact on benthic communities in the Gulf of Maine within the past two decades. Prior to the 1970s, the climax community in the shallow subtidal was composed of Laminaria spp. kelp beds with an understory of arborescent red algae. In the 1980s, a population explosion of the green sea urchin, Strongylocentrotus droebachiensis, created an alternate community state, urchin barrens. Recently, a new community has been observed in former urchin barrens and kelp beds. This assemblage is principally composed of the introduced species: Codium fragile subsp. tomentosoides (green alga), Membranipora membranacea (bryozoan), Diplosoma listerianum (tunicate), Bonnemaisonia hamifera (red alga) and the opportunistic species Mytilus edulis (mussel) and Desmarestia aculeata (brown alga). In addition to changes in relative abundance, many of these species have greatly expanded their distribution and habitat selection. A model detailing mechanisms for the transition of the traditional kelp bed and urchin barren communities to others is presented and implications for this new community are discussed. This revised version was published online in July 2006 with corrections to the Cover Date.