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.     

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.     

Random movement pattern of the sea urchin Strongylocentrotus droebachiensis

We describe the fine-scale movement of the sea urchin Strongylocentrotus droebachiensis based on analyses of video recordings of undisturbed individuals in the two habitats which mainly differed in food availability, urchin barrens and grazing front. Urchin activity decreased as urchin density increased. Individuals alternated between moving and being stationary and their behaviour did not appear to be affected by either current velocity (within the range from 0 to 15 cm s^− 1) and temperature (2.3 to 6.0 °C). Movement of individuals at each location was compared to that predicted by a random walk model. Mean move length (linear distance between two stationary periods), turning angle and net squared displacement were calculated for each individual. The distribution of turning angles was uniform at each location and there was no evidence of a relationship between urchin density and either move length or urchin velocity. The random model predicted a higher dispersal rate at locations with low urchin densities, such as barrens habitats. However, the movement was sometimes greater or less than predicted by the model, suggesting the influence of local environmental factors. The deviation of individual paths from the model revealed that urchins can be stationary or adopt a local (displacement less than random), random or directional movement. The net daily distance displaced on the barrens, predicted by a random walk model, was similar to the observed movement recorded in our previous study of tagged urchins at one site, but less than that observed at a second site. We postulate that the random dispersal of urchins allows individuals on barrens to reach the kelp zone where food is more abundant although the time required to reach the kelp zone may be considerable (months to years). Urchins decrease their rate of dispersal once they reach the kelp zone so that they likely remain close to this abundant food sources for long periods.     

A COMPARISON OF DRAGON KELP,EUALARIA FISTULOSA, (PHAEOPHYCEAE) FECUNDITY IN URCHIN BARRENS AND NEARBY KELP BEDS THROUGHOUT THE ALEUTIAN ARCHIPELAGO1

The Aleutian Archipelago coastal ecosystem has undergone a dramatic change in community composition during the past two decades. Following the removal of ～99% of the sea otters, Enhydra lutris, from the ecosystem, changes to the benthic communities resulted in widespread losses to most of the region’s kelp beds and corresponding increases in the prevalence of urchin barrens. Within the urchin barrens, the few kelps that have remained are exposed to elevated light, nutrients and currents, all of which may enhance their physiological condition and thus result in greater fecundity. To explore this further, we examined patterns of sporophyte fecundity in the dominant canopy‐forming kelp, Eualaria fistulosa, in both urchin barrens and in nearby kelp beds at seven Aleutian Islands spanning a range of 800 km. We found that the average weight of E. fistulosa sporophyll bundles was significantly greater on sporophytes occurring in the urchin barrens than in the nearby kelp beds. Furthermore, the average number of zoospores released per cm² of sporophyll area was also significantly greater in individuals from the urchin barrens than the nearby kelp beds. When these two metrics were combined, our results suggest that individual E. fistulosa sporophytes occurring in the urchin barrens may produce as many as three times more zoospores than individual E. fistulosa sporophytes occurring in the nearby kelp beds, and thus they may contribute disproportionately to the following year’s sporophyte recruitment in both urchin barrens and the adjacent kelp beds.     

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.     

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.     

Predation cues rather than resource availability promote cryptic behaviour in a habitat-forming sea urchin

It is well known that predators often influence the foraging behaviour of prey through the so-called “fear effect”. However, it is also possible that predators could change prey behaviour indirectly by altering the prey’s food supply through a trophic cascade. The predator–sea urchin–kelp trophic cascade is widely assumed to be driven by the removal of sea urchins by predators, but changes in sea urchin behaviour in response to predators or increased food availability could also play an important role. We tested whether increased crevice occupancy by herbivorous sea urchins in the presence of abundant predatory fishes and lobsters is a response to the increased risk of predation, or an indirect response to higher kelp abundances. Inside two New Zealand marine reserves with abundant predators and kelp, individuals of the sea urchin Evechinus chloroticus were rarer and remained cryptic (i.e. found in crevices) to larger sizes than on adjacent fished coasts where predators and kelp are rare. In a mesocosm experiment, cryptic behaviour was induced by simulated predation (the addition of crushed conspecifics), but the addition of food in the form of drift kelp did not induce cryptic behaviour. These findings demonstrate that the ‘fear’ of predators is more important than food availability in promoting sea urchin cryptic behaviour and suggest that both density- and behaviourally mediated interactions are important in the predator–sea urchin–kelp trophic cascade.     

Importance of spatial population characteristics on the fertilization rates of sea urchins

We show that inclusion of population characteristics in coupled advection-diffusion and fertilization-kinetics models results in higher fertilization rates than those previously reported in theoretical studies. We incorporate parameters related to both individuals and populations by running simulations over a large spatial scale and incorporating sperm contribution from multiple males. We compare predictions for three subpopulations of the sea urchin Strongylocentrotus droebachiensis (those occupying kelp beds, barrens, and grazing fronts) to observations from small-scale experiments, and estimate effects of population size and current velocity in each subpopulation. Model outputs suggest that fertilization rates are low in kelp beds, intermediate in barrens, and high in grazing fronts. In all populations, increasing current velocity has a negative effect on the relationship between fertilization rate and downstream distance of gametes after release, but no effect on the relationship between fertilization rate and elapsed time since gamete release. Our model output was most sensitive to changes in the number of spawning males and the sperm release rate, suggesting that spawning synchrony and high gonadic index could greatly increase the fertilization success in sea urchins.     

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.     

Effects of removing sea urchins (Strongylocentrotus droebachiensis): Stability of the barren state and succession of kelp forest recovery in the east Atlantic

Stability properties of the barren state of a kelp forest-sea urchin system were studied in northern Norway. The ability of the sea urchin Strongylocentrotus droebachiensis to maintain high population densities and recover from perturbations, and the succession of kelp forest revegetation, were studied experimentally by reducing the sea urchin density on a barren skerry. Additional information was obtained from community changes following a natural, but patchy, sea urchin mortality that varied between sites. On the barren grounds, high sea urchin densities (30 50 per m²) is maintained by annual recruitment. Severe reductions of sea urchin densities initiated luxuriant kelp growth, while more moderate reductions allowed establishment of opportunistic algae (during spring and early summer), but no kelps. Succession of algal growth, after the severe decline in sea urchin densities, followed a predictable pattern. At first the substrate was colonized by filamentous algae, but within few weeks they were outcompeted by the fast growing kelp Laminaria saccharina. After 3–4 years of the removal experiment, the slower-growing, long-lived kelp L. hyperborea became increasingly dominant. Increased food availability after reduction in sea urchin density led to increased individual growth of the remaining sea urchins. However, the population density did not increase, neither from recruitment nor immigration from adjacent areas with high sea urchin densities. Possibly, early establishment of a dense kelp stand, may represent a breakpoint in the ability of sea urchins to reestablish a barren state. The ability of L. saccharina quickly to invade and monopolize an area may have both positive and negative effects on the succession towards the climax L. hyperborea kelp forest. Competitive interactions may slow the process, but development of a dense stand of L. saccharina will also reduce grazing risk on scattered recruits of the more slowly growing L. hyperborea.     

Detached kelps from distant sources are a food subsidy for sea urchins

Trophic subsidies link habitats and can determine community structure in the subsidised habitats. Knowledge of the spatial extents of trophic interactions is important for understanding food webs, and for making spatial management practices more efficient. We demonstrate trophic linkages between detached (drift) fragments of the kelp Ecklonia radiata and the purple sea urchin Heliocidaris erythrogramma among discrete rocky reefs separated by kilometres. Sea urchins were abundant at one inshore reef, where the biomass of drift was usually high. There, sea urchins trapped detached kelp at high rates, although local kelp abundance was low. Most detached kelp present on the reef was retained by sea urchins. Detached seagrass, which was abundant on the reef, was not retained by sea urchins in large quantities. Experiments with tethered pieces of kelp showed that sea urchins only consumed detached fragments, and did not consume attached kelps. Comparisons of the morphology of detached fragments of kelp collected from the inshore reef to attached kelps from reefs further offshore showed that a large proportion (30-95%, varying among dates) of the fragments originated at distant reefs (>/=2 km away). At the inshore reef, the sea urchin H. erythrogramma is subsidised by detached kelps, and detached kelp fragments have been transported across landscapes. Cross-habitat resource subsidies therefore link discrete reef habitats separated by kilometres of non-reef habitat.     

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.     

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.     

Predator‐induced selection on urchin activity level depends on urchin body size

Temporally consistent individual differences in behavior impact many ecological processes. We simultaneously examined the effects of individual variation in prey activity level, covering behavior, and body size on prey survival with predators using an urchin–lobster system. Specifically, we tested the hypothesis that slow‐moving purple sea urchins (Strongylocentrotus purpuratus) and urchins who deploy extensive substrate (pebbles and stones) covering behavior will out‐survive active urchins that deploy little to no covering behavior when pitted against a predator, the California spiny lobster (Panulirus interruptus). We evaluated this hypothesis by first confirming whether individual urchins exhibit temporally consistent differences in activity level and covering behavior, which they did. Next, we placed groups of four urchins in mesocosms with single lobster and monitored urchin survival for 108 hr. High activity level was negatively associated with survival, whereas urchin size and covering behavior independently did not influence survival. The negative effect of urchin activity level on urchin survival was strong for smaller urchins and weaker for large urchins. Taken together, these results suggest that purple urchin activity level and size jointly determine their susceptibility to predation by lobsters. This is potentially of great interest, because predation by recovering lobster populations could alter the stability of kelp forests by culling specific phenotypes, like foraging phenotypes, from urchin populations.     

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.     

Disturbance and organization of macroalgal assemblages in the Northwest Atlantic

Large seaweeds are often structurally dominant in subtidal and intertidal rocky shore benthic communities of the N.W. Atlantic. The mechanisms by which these algal assemblages are maintained are surprisingly different in the two habitats. In the subtidal community, kelps are dominant space competitors in the absence of strong grazing interactions. In contrast, the large perennial seaweeds of intertidal zones (fucoids and Chondrus crispus) are competitively inferior to both sessile filter feeders and ephemeral, pioneer algal species. Intertidal seaweed beds are maintained by carnivory of whelks, which reduces filter feeder populations, and by herbivorous periwinkles which reduce ephemeral algal populations. Through most of the intertidal zone, disturbance, both biological and physical, dictates which species shall compete and equilibrium conditions obtain subsequently.The roles of subtidal consumers are quite different. Sea urchins are the major algal herbivores and these voracious animals maintain an equilibrium state in which large tracts of subtidal coralline pavement are kept free of kelp forests. Urchins do not seem to play a successional facilitative role for kelps in the way that periwinkles do for fucoids in the intertidal. Control of herbivore populations is thus a key to the maintenance of subtidal foliose algal beds. It is clear that parasitic amoebas can decimate sea urchin populations so that kelp forest dominance is assured. However, the importance of carnivory in limiting urchins in the subtidal community is unclear in the absence of appropriate manipulation experiments. It is possible that carnivorous decapods and fin fish control sea urchin populations and hence foliose algal abundance, but this must remain speculative. The seaweed-dominated state of the subtidal system is an alternative equilibrium condition to the urchin/coralline alga configuration. The structure of the kelp beds is relatively uniform in responding to frequent small-scale, infrequent large-scale, or no, disturbance.     

Echinoderms display morphological and behavioural phenotypic plasticity in response to their trophic environment

The trophic interactions of sea urchins are known to be the agents of phase shifts in benthic marine habitats such as tropical and temperate reefs. In temperate reefs, the grazing activity of sea urchins has been responsible for the destruction of kelp forests and the formation of 'urchin barrens', a rocky habitat dominated by crustose algae and encrusting invertebrates. Once formed, these urchin barrens can persist for decades. Trophic plasticity in the sea urchin may contribute to the stability and resilience of this alternate stable state by increasing diet breadth in sea urchins. This plasticity promotes ecological connectivity and weakens species interactions and so increases ecosystem stability. We test the hypothesis that sea urchins exhibit trophic plasticity using an approach that controls for other typically confounding environmental and genetic factors. To do this, we exposed a genetically homogenous population of sea urchins to two very different trophic environments over a period of two years. The sea urchins exhibited a wide degree of phenotypic trophic plasticity when exposed to contrasting trophic environments. The two populations developed differences in their gross morphology and the test microstructure. In addition, when challenged with unfamiliar prey, the response of each group was different. We show that sea urchins exhibit significant morphological and behavioural phenotypic plasticity independent of their environment or their nutritional status.     

Fauna associated with detached kelp in different types of subtidal habitats of the White Sea

The fauna, associated with Laminaria and other largebrown macroalgae was studied by using SCUBA anddredging in two different types of underwater habitatsof the White Sea.In shallow water fjords and bays, with a depth of nomore than 30–40 m, detached kelp (mainly Laminaria saccharina, L. digitata and Alaria esculenta) formed large accumulations. One ofthese benthic accumulations, which has existed morethan 20 years, was studied. It covers about2000 m², and is about 2 m thick. The upper layerof the accumulation of fronds is characterized by highturbulence and is well aerated. The lower layer ischaracterized by anoxic conditions. Mats of sulphurbacteria were not observed, although fronds in themiddle layer were covered by layers of cyanobacteria.About 50 species of macroinvertebrates were found,mainly species that are normally associated withliving kelp, such as the detritivorous species Ophiura robusta and Gammarus oceanicus, and fewspecies that are specific inhabitants of organic-richbiotopes in the White Sea such as Capitellacapitata, Ophryotrocha irinae and Nebaliabipes. It was remarkable that in the shallow waterbasins of the White Sea, the process of decompositionof brown algae in the sublittoral takes place withoutsea urchins, and no other macrofaunal form plays anecological role in the mechanical breakdown of theplant substratum, even not in the large accumulationsof detached kelp.Along the open rocky shoreline, communities associatedwith dead detached kelp were situated at a depth of60–90 m, 40–50 m below the belt of living kelp. Inthis deep zone, no macroinvertebrates typical of thekelp community in the photic zone were found. Duringthe passage from the shoreline to the deeper benthiccommunity, where sea urchins were dominant, all plantdebris became fragmented. These deeper benthiccommunities appeared to be the zone for decompositionof the detached kelp.     

Accounting for size‐specific predation improves our ability to predict the strength of a trophic cascade

Predation can influence the magnitude of herbivory that grazers exert on primary producers by altering both grazer abundance and their per capita consumption rates via changes in behavior, density‐dependent effects, and size. Therefore, models based solely on changes in abundance may miss key components of grazing pressure. We estimated shifts in grazing pressure associated with changes in the abundance and per capita consumption rates of sea urchins triggered by size‐selective predation by sea otters (Enhydra lutris). Field surveys suggest that sea otters dramatically decreased the abundance and median size of sea urchins. Furthermore, laboratory experiments revealed that kelp consumption by sea urchins varied nonlinearly as a function of urchin size such that consumption rates increased to the 0.56 and 0.68 power of biomass for red and green urchins, respectively. This reveals that shifts in urchin size structure due to size‐selective predation by sea otters alter sea urchin per capita grazing rates. Comparison of two quantitative models estimating total consumptive capacity revealed that a model incorporating shifts in urchin abundance while neglecting urchin size structure overestimated grazing pressure compared to a model that incorporated size. Consequently, incorporating shifts in urchin size better predicted field estimates of kelp abundance compared to equivalent models based on urchin abundance alone. We provide strong evidence that incorporating size‐specific parameters increases our ability to describe and predict trophic interactions.     

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.