This review about the genus Laminaria sensu lato summarizes the extensive literature that has been published since the overview of the genus given by Kain in 1979. The recent proposal to divide the genus into the two genera Laminaria and Saccharina is acknowledged, but the published data are discussed under a 'sensu lato' concept, introduced here. This includes all species which have been considered to be 'Laminaria' before the division of the genus. In detail, after an introduction the review covers recent insights into phylogeny and taxonomy, and discusses morphotypes, ecotypes, population genetics and demography. It describes growth and photosynthetic performance of sporophytes with special paragraphs on the regulation of sporogenesis, regulation by endogenous rhythms, nutrient metabolism, storage products, and salinity tolerance. The biology of microstages is discussed separately. The ecology of these kelps is described with a focus on stress defence against abiotic and biotic factors and the role of Laminaria as habitat, its trophic interactions and its competition is discussed. Finally, recent developments in aquaculture are summarized. In conclusion to each section, as a perspective and guide to future research, we draw attention to the remaining gaps in the knowledge about the genus and kelps in general.

https://www.tandfonline.com/doi/pdf/10.1080/09670260701711376

The genus Laminaria sensu lato : recent insights and developments

The mass mortality of the echinoidDiadema antillarum Philippi in 1983/1984 resulted in dramatic changes in the benthic algal community. This study reports data on the population densities of sea urchins prior to and following the mass mortality and data on the biomass, species composition, and rates of primary productivity of algal communities at several study sites in St. Croix, U.S. Virgin Islands. The mass mortality reducedD. antillarum population densities by 95 to 99%. Population densities were reduced further by a second, less severe mortality event in October 1985. Over a period of 2 yr, recruitment ofD. antillarum larvae to the study sites was low and population densities remained at 2 to 30% of their premortality levels. The effects of the mass mortality on the algal community were significant. Algal biomass increased by 22 to 439% across reef zones 16 mo after the die-off. Rates of primary productivity per unit biomass (chlorophylla) remained at approximately 60% of premortality levels in shallow-reef habitats for 25 mo following the die-off. The magnitude of the response of the algal community was positively correlated with the previous population densities ofD. antillarum. The species composition of the algal community also shifted dramatically. Prior to the mass mortality, algal communities were dominated by algal turfs and crustose algae, and macroalgae were either rare or absent. Twenty-five months after the mass mortality, algal turfs covered 40% of the area and macroalgae covered 47%. Many of the macroalgal species are not consumed by herbivores and much of the algae is dislodged by storm waves and is exported from the reef. This represents a transition from a community dominated by a grazing-based food web to one where the majority of primary production may be exported to adjacent communities.

Mass mortality ofDiadema antillarum

The sea urchin Centrostephanus rodgersii is important in determining biogenic habitat structure on rocky reefs in New South Wales. Individual C. rodgersii, in common with other diadematids, are found in shelters during the day, from which they emerge to forage at night and maintain patches of barrens habitat. The effect of availability of shelter on the foraging behavior of C. rodgersii, and on the abundance of benthic invertebrates and algae, was investigated by a field experiment. Physical heterogeneity in the form of large boulders were translocated between the barrens and Ecklonia forest habitats and arranged so as to either maximize or minimize shelter. The fauna and flora on translocated boulders became progressively similar to that found in the surrounding habitat. Most obvious of these changes were reductions in the density of foliose algae on boulders moved from Ecklonia forest to barrens and reductions in the density of limpets in the reverse translocation. C. rodgersii recruited to all treatments irrespective of where the boulders came from, where they were moved to, and the amount of shelter provided. The availability of shelter was clearly a sufficient condition for the creation of areas of barrens habitat. The reliance on shelter by these sea urchins makes their local distribution and, therefore, that of the barrens habitat, more predictable than in other temperate regions.

Spatial Heterogeneity, Sea Urchin Grazing, and Habitat Structure on Reefs in Temperate Australia

Although human-mediated extinctions disproportionately affect higher trophic levels, the ecosystem consequences of declining diversity are best known for plants and herbivores. We combined field surveys and experimental manipulations to examine the consequences of changing predator diversity for trophic cascades in kelp forests. In field surveys we found that predator diversity was negatively correlated with herbivore abundance and positively correlated with kelp abundance. To assess whether this relationship was causal, we manipulated predator richness in kelp mesocosms, and found that decreasing predator richness increased herbivore grazing, leading to a decrease in the biomass of the giant kelp Macrocystis. The presence of different predators caused different herbivores to alter their behaviour by reducing grazing, such that total grazing was lowest at highest predator diversity. Our results suggest that declining predator diversity can have cascading effects on community structure by reducing the abundance of key habitat-providing species.

/pdf/predator-diversity-strengthens-trophic-cascades-in-kelp-3cwweyysi9.pdf

Predator diversity strengthens trophic cascades in kelp forests by modifying herbivore behaviour

There are two alternate community states in the rocky subtidal of the Atlantic
coast of Nova Scotia, an unproductive sea urchin/coralline alga community, and highly
productive kelp beds dominated by Laminaria longicruris. Disease-induced mortality of
the sea urchins (Strongylocentrotus droebachiensis) triggered a switch from the first state
to the second and provided a unique opportunity to study (1) the ability of L. longicruris
to recover its former dominant status, and (2) its stability when competing with other
seaweeds and when perturbed by storms and grazers other than urchins. Rates of recolo-
nization of L. longicruris depended on the proximity of a refugial source of spores. When
reproductive plants were nearby, a closed canopy developed within 18 mo of urchin mortality. When a reproductive population was several kilometres away, there was sparse
recolonization for 3 yr, then a massive recruitment occurred with closure of the canopy in
the 4th yr.
Laminaria is clearly the competitive dominant in the seaweed community. Manipulative experiments showed that the kelp limits the abundance of several understory species,
but there was no evidence that the abundant annual seaweeds limited kelp recruitment.
When sea urchins were rare, the density and growth rates of Laminaria were influenced
mostly by intraspecific competition. When the canopy of adult plants was removed there
was a dramatic increase in kelp recruitment, but the recruits that grew in dense patches in
the clearings were significantly smaller than those of a similar age that grew more sparsely
beneath the canopy. Once the kelp recovered from destructive grazing and formed a mature
forest, it was able to maintain its dominance, even in habitats subject to severe nutrient
stress for 8 mo of the year. For most ofthe year mortality and erosion of laminae outweighed
the effects of recruitment and growth, and the canopy declined, especially during winter
when storms were frequent. Erosion was exacerbated by grazing of the gastropod Lacuna
vincta. However, in late winter and early spring, recruitment and rapid growth restored
the canopy. When severe storm damage was simulated by completely removing Laminaria
in patches, the kelp rapidly recolonized and soon outgrew other seaweeds.
Unlike the competitive dominants in kelp bed systems in the northeast Pacific, L .
longicruris in Nova Scotia manifests multiple patterns of adaptation that enable it to
dominate early and late stages of succession in a range of habitats of different levels of
nutrient stress and of disturbance from storms and grazers. The principal threat to the
stability of the kelp beds is destructive grazing by sea urchins. We suggest that the consid-
erable differences between the dynamios of kelp beds in Nova Scotia and those of the
northeast Pacific, and the high degree of stability of L . longicruris stands in Nova Scotia,
is attributable to the low diversity of kelps and therefore low levels of competition in Nova
Scotia, and to the multiple adaptations of L . longicruris that enable it to tolerate several
stresses and disturbances.
We argue that the dynamics of community organization, and therefore the stability
properties of this system are determined primarily by biological interactions and not by
physical variables. This differs from the kelp communities in the northeast Pacific, in which
both biological and physical factors influence dynamics significantly at a primary level.
We offer a qualitative model of the dynamics of community structure in Nova Scotia that may be viewed as a set of deterministic "subroutines," in which each subroutine describes
the outcome of a particular biological interaction. The subroutine(s) that predominate at
one point in time and space are probably determined mostly by physical hydrographic
variables that have a large stochastic component.

Diversity, Patterns of Adaptation, and Stability of Nova Scotian Kelp Beds

Defensive aggregation by Strongylocentrotus droebachiensis has been invoked as the fulcrum for the transformation of macroalgal beds into coralline barrens in the northwest Atlantic. We critically examined some of the mechanisms contributing to aggregation behavior by experimentally manipulating sea urchins, purported predators and food during autumn 1983 and spring 1984 both in the laboratory and field. We utilized several approaches to examine a range of sea urchin responses to the presence of food, tethered predators, caged predators, crushed con-specifics in the field and predators in laboratory tanks. Some of the field cages had the property of allowing free passage to sea urchins while retaining lobsters; this allowed distinctions to be made between artifacts caused by cage walls or topographic barriers and unrestricted behavioral responses of sea urchins. The results falsified the hypothesis that aggregations of S. droebachiensis are elicited by predators. Except in the presence of algae, sea urchins always avoided decapod predators (but not sea stars) and fled from the vicinity of injured conspecifics. However, avoidance behavior was subordinate to feeding responses, demonstrated by mass migrations of sea urchins into cages (with lobsters) to feed on algae. We reaffirmed by observation and manipulation previous studies which showed that sea urchins aggregate only in the presence of food. Two types of sea urchin groupings were delineated: (1) surficial two dimensional associations, often caused by topographic or other features which inhibited dispersal and (2) cohesive three-dimensional aggregations induced by food.

I. G. Babb

Papers

Experimental evaluation of aggregation behavior in the sea urchin Strongylocentrotus droebachiensis: A reinterpretation