Thomas Wernberg

Export of detached macroalgae from reefs to adjacent seagrass beds

Trophic linkages across habitats are thought to be strong when areas of different productivity are juxtaposed. Reefs dominated by macroalgae are commonly juxtaposed to less productive seagrass beds. We tested if macroalgae detached from 12 rocky reefs in south-western Australia were exported to adjacent seagrass beds and consumed by seagrass-associated fauna. We also assessed the extent of linkages by testing for patterns in biomass and consumption of reef algae, and density of herbivorous fish with increasing distance away from reefs.Detached reef algae were found in seagrass beds adjacent to all reefs. The biomass varied among reefs and with distance from reef, but detached reef algae within the seagrass beds comprised up to 23% (mean 3.6% ± 0.7 SE) of attached algae growing on an equivalent area of reef. Maximum accumulations were found immediately adjacent to reefs (0xa0m) and at the furthest distance away (>300xa0m). Kelp (Ecklonia radiata) dominated the attached and detached algae, and up to 77% of the biomass of E. radiata tethered in seagrass beds were consumed over 5xa0days (mean 11.7% ± 0.5 SE). There were more herbivorous fish at 0xa0m than at >300xa0m away from reefs, and consumption of tethered kelp was typically highest at 0xa0m, but was in some cases highest at >300xa0m.Our study documents that, over hundreds of kilometres of coastline, macroalgae are exported from reefs to adjacent seagrass beds where they are consumed by seagrass-associated fauna. While reef algae in seagrass beds may be a patchy resource at a single time, at landscape scales and over longer time periods, the supply will be relatively predictable. We therefore suggest that detached reef algae form a significant trophic link between reefs and seagrass beds, and that this trophic link extends to distances of at least hundreds of metres away from individual reefs.

Epibiota communities of the introduced and indigenous macroalgal relatives Sargassum muticum and Halidrys siliquosa in Limfjorden (Denmark)

Sargassum muticum (Phaeophyceae, Fucales) has recently been introduced to Limfjorden (Denmark) where its closest relative is the indigenous Halidrys siliquosa. Previous studies have demonstrated large quantitative (canopy biomass) and qualitative (canopy persistence) differences in the habitat available to epibiota within the canopies of these two macroalgae. We therefore hypothesised that these algae would support different epibiota communities and tested this by sampling the epibiota of S. muticum and H. siliquosa on seven occasions throughout 1997 by enclosing entire thalli in mesh bags. We found 53 epibiota taxa and, with only one exception, they were all recorded on both host species. Species richness and abundance of epibiota exhibited clear seasonal variation on both host species, although epibiota biomass was seasonally constant on H. siliquosa but not on S. muticum. These patterns were consistent with the different life histories of the host species. There was a weakly negative correlation between thallus size and epibiota biomass for both host species. When taking species-specific seasonal variation in thallus size into consideration, S. muticum and H. siliquosa were found to support significantly different epibiota biomasses. Multivariate analyses showed that epibiota community structure was different, although highly overlapping, between the two species, whereas there was an almost parallel temporal development in epibiota community structure. We conclude that it is unlikely that the introduction of S. muticum to Limfjorden has caused major changes in local epibiota community structure. However, the standing stock of epibiota is likely to have increased.

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 Eckloniaxa0radiata and the purple sea urchin Heliocidarisxa0erythrogramma 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 (≥2xa0km away). At the inshore reef, the sea urchin H.xa0erythrogramma 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.

Spatio-temporal distribution patterns of the invasive macroalga Sargassum muticum within a Danish Sargassum-bed

Sargassum muticum was first observed in Scandinavia in Limfjorden (Denmark) in 1984, where it is now the most abundant and conspicuous macroalga. Despite the ecological importance of Sargassum, few studies have described seasonal patterns within Scandinavian Sargassum beds. We quantified the dynamics of macroalgae among years and seasons along a depth transect through a typical Sargassum bed in Limfjorden. The annual investigations (summer transects 1989–1999) showed a gradual increase in the dominance of Sargassum, especially at the 2–4-m depth interval. Significant seasonal dynamics in macroalgal abundance and assemblage structure were observed in this depth interval; the mean cover of Sargassum varied from ca. 5% (autumn and winter) to 25% (mid-summer). In comparison, encrusting algae had high and relatively stable covers throughout the year (ca. 20%). Other perennial macroalgae had low mean covers (<2%) characterized by a few patches of higher abundances. Except from a spring bloom, filamentous algae had low covers throughout the year. Within this relatively uniform bed, Sargassum abundance was positively related to boulders >10xa0cm in diameter and species richness was negatively correlated to depth and stones <10xa0cm in diameter, and non-correlated to other algal form-groups or grazer densities. Thus, in Limfjorden, the distribution of Sargassum is determined by large- (>6xa0m) and small-scale (<1xa0m) depth differences where low light limits Sargassum at depth, physical disturbance and sediment stress limits Sargasum in shallow waters, and the presence of stable boulder substratum facilitate Sargassum. Competition for space from other macroalgae and herbivory are probably of minor importance.

Miniview: What affects the forces required to break or dislodge macroalgae?

The forces required to break or dislodge benthic macroalgae can be linked to many important ecological processes such as survival, transport and dispersal, and it is, therefore, important to understand what factors affect these forces. We review existing studies, using simple vote-counting, to establish what ecological factors have been found to influence macroalgal break forces. We also used the compiled information to test for the existence of a general allometric relationship between thallus area and break force. We found 30 studies reporting break forces for 27 species of macroalgae from 118 populations. Species within the orders Laminariales, Fucales and Gigartinales were well studied whereas other orders had been neglected. Two-thirds of all studies were from rocky intertidal habitats and two-thirds were from North American coastlines. The most frequently tested ecological factor was wave exposure (14 studies with 7 significant effects). Other factors that were found to affect break force significantly in at least one study included time of sampling, thallus size, substratum and species. Generally, most factors had been addressed only a few times, emphasizing a need for further study. We found a highly significant positive relationship between thallus size and break force across a wide range of species and ecological conditions, providing evidence that size alone is an important determinant of macroalgal break force. To strengthen inferences and ensure that meta-analyses can be conducted, we suggest that future studies (i) use nested replication in space and time, (ii) include taxa and regions that have not been studied in detail, and (iii) report, as a minimum, corresponding information on break forces, thallus sizes, substratum type, wave exposure and time of sampling.

Alien macroalgae in Denmark - a broad-scale national perspective.

Abstract Most studies documenting the importance of alien macroalgae relative to native species are based on smaller-scale sampling programmes. Between 1989 and 2003, a Danish monitoring programme collected data on the percentage cover of macroalgae at more than 600 locations throughout the country. We examined this data set to estimate the relative abundance of alien species in the algal flora on large spatiotemporal scales, i.e. across depth ranges, regions and years. Of the 10 alien macroalgal species known to inhabit Danish coastal waters, nine were found in the survey. Most of the alien species were only present in low quantities (<1% of the entire flora). In contrast, the two most common alien species, Sargassum muticum and Bonnemaisonia hamifera, constituted 2–7% of the assemblages, depending on depth, region and year. Sargassum muticum was abundant from 0 to 5 m in the northwestern region, where salinity and species richness are highest, whereas B. hamifera was abundant in several regions in deeper waters, where the native flora is species-poor. Based on their relatively high abundance, we hypothesize that these two aliens have had the largest impact on the native communities. Of some concern is the recent introduction of Gracilaria vermiculophylla. This species has traits that match the conditions of Danish estuaries and may become widespread with potential negative impacts on native biota.

Scale of impact determines early post-disturbance assemblage structure in subtidal Fucus beds in the Baltic Sea (Bornholm, Denmark)

Disturbances occasionally create gaps of varying sizes in the canopy cover of macroalgae on temperate reefs. This study quantified the size distribution of naturally occurring gaps in a subtidal Fucus serratus canopy, and experimentally tested the hypotheses that (i) early post-disturbance abundance of dominant assemblage-formers in the canopy-dominated habitat will depend on the spatial extent of the impact, and (ii) the effects of disturbance on the abundance of dominant assemblage-formers at the edges of large impacted areas will be similar to the effects at small impacted areas. Gaps of up to 6u2009m diameter accounted for 17% of the habitat and, while small gaps were numerically dominant, all gap size-classes accounted for a similar proportion of habitat. Experimental disturbances (canopy clearing) ranging from 0.6 to 4.8u2009m diameter caused consistent scale-dependent divergence of assemblage structure after 21 days. Of particular interest was a dense cover (∼75%) of filamentous algae in the centres of large (≥1.2u2009m) clearings. Filamentous algae are widely known to have negative effects on Fucus recruitment and growth, and their rapid increase in abundance suggests that future recovery of the Fucus canopy could be adversely affected. The abundance of most taxa at the edges of large clearings was either indistinguishable from abundances in small clearings and intact canopy, or intermediate between abundances in these treatments and in the centres of large clearings. Juvenile Fucus were, however, more abundant around edges than anywhere else. These experimental outcomes support the model that scale-dependent divergence of assemblage structure is conveyed by an escape from processes associated with edges of the original habitat. I propose that, in the subtidal Fucus beds of Bornholm, recovery from large disturbances is restricted to encroachment from the edges because the adjacent canopy suppresses the growth of filamentous algae and thus facilitates regeneration and recruitment of Fucus.

Global warming is eroding the resilience of kelp beds

Global warming’s effect on the phenology and distribution of biota threatens the preservation of biodiversity and ecosystem services worldwide. A critical element for conservation success in the future is understanding the link between physiological and ecological responses of key species to multiple concurrent environmental changes. Current evidence for how ocean temperature affects the biota in socio-economically important marine habitats is scant, correlative and provides little causal information on likely synergistic effects of multiple perturbations. Here, we use a comparative experimental approach to demonstrate that, in Australasian kelp beds, physiological adjustment to maintain abundance in a warm ocean climate reduces the ecological competency of juvenile kelp, and that this suppresses the capacity for canopy recovery from physical disturbances of increasing intensity. The implied erosion of resilience is mediated by a shift in adult-juvenile interactions from competitive under cool (benign) to facilitative under warm (adverse) conditions. This supports recent theoretical predictions that positive interactions among organisms will become increasingly important to the maintenance of ecological function in a warmer future. These results demonstrate that, rather than gradually shifting range boundaries pole-ward, the commonly predicted response to global warming, patterns of abundance may be maintained through physiological adjustment while the underlying ecological processes change. The combined effect of ocean warming and increasing disturbances from extreme events will therefore bring kelp beds closer to a threshold where persistent loss of habitat and ecological function will occur if they are subjected to additional stressors (e.g., reduced water quality). Climate Change: Global Risks, Challenges and Decisions IOP Publishing IOP Conf. Series: Earth and Environmental Science 6 (2009) 312008 doi:10.1088/1755-1307/6/1/312008