Variations in the dispersal curves of macroalgal propagules from a source

The process of dispersal is critical to marine benthic species (i.e. invertebrates and algae) as a fundamental element of population ecology and a crucial ecological process that maintains the diversity in communities. We simultaneously sampled the abundance of spores inhabiting the water column at different distances from known parent sources at 3 sites along the coast of central Chile. From these data we constructed 258 dispersal curves for common rocky intertidal macroalgae. Only 43.8% of these curves could be predicted by the expected model, which describes the spatial distribution of propagules to be dominated by a larger concentration near the parent individual or “source site”, followed by a marked decrease in abundance with increasing distance. The curves that departed from the expected model (56.2%) were grouped into three curve types, according to the number of propagule abundance maxima observed in space. This work suggests that macroalgal propagule dispersal patterns are more variable than previously thought. The existence of several alternative curves to the expected model, as well as the presence of one to several abundance maxima associated with the differential distribution of propagule patches in the water column, suggests the idea that propagules are released in pulses which can be transported variable distances from the source site.     

An integrated study of the temporal and spatial variation in the supply of propagules, recruitment and assemblages of intertidal macroalgae on a wave-exposed rocky coast, Victoria, Australia

Recruitment is known to influence distributions and abundances of benthic marine organisms. It is therefore important to document patterns of variability in recruitment and how these relate to patterns in established assemblages. This study provides an integrated assessment of the temporal and spatial variation in supply and recruitment of propagules and established populations of several macroalgae. Propagules in water samples from two stages of the incoming tide, recruitment to artificial substrata and percentage cover of species established on the shore were recorded every 2 months from December 1994 to October 1995, in two zones of an intertidal, wave-exposed rocky shore. Variability in recruitment was measured at three spatial scales: 10s cm, 100s cm and 100s m. Availability and recruitment of most taxa were greatest between April and August, although many species had available propagules and recruited throughout the year. Temporal variation in the established assemblages was, however, more species-specific. Differences in established assemblages between zones were reflected in differences in availability and recruitment of propagules between zones. Recruitment could not be predicted directly from supply of propagules, but the two processes were linked. For most species, the greatest variation in recruitment occurred at the smallest spatial scale of 10s cm, although there was also considerable large-scale (between site) variation in recruitment of several species. Results indicate that while pre-and post-settlement mortality are likely to influence macroalgal distribution and abundance, the temporal and spatial variability in supply and recruitment of propagules can explain much of the patchiness in macroalgal assemblages.     

Bottom-up and cascading top-down control of macroalgae along a depth gradient

On marine rocky shores, macroalgal herbivory is often intense, such that the cascading effects of fish predation may contribute to the control of algal communities. To estimate the magnitudes of top-down and bottom-up control on a macroalgal community, we manipulated the access of carnivorous fish to macroalgal colonization substrates, as well as nutrient availability, at two sub-littoral depths. There were three levels of fish manipulation: natural fish community, no fish and the enclosure of one common species, the perch, Perca fluviatilis. We found a clear cascade effect of fish predation on both the total density and several individual species of macroalgae, which was more pronounced in deep than shallow water. The density of the dominant grazers, i.e. snails, increased in nutrient-enriched conditions; perch were inefficient in controlling herbivores, and had therefore no cascading effect on algal densities under such conditions. Although nutrients enhanced the growth of opportunistic algae, herbivores, in the absence of fish, inhibited this response. While algal diversity was higher in shallow than in deep water, the enrichment effect was opposite at the two depths with lowered diversity in the shallows and increased at depth. Our results indicate that fish predation is an efficient regulator of meso-herbivores and that its effect thereby cascades onto the producer trophic level such that both perennial and opportunistic algae benefit from the presence of fish. This cascade effect is probably stronger at depth where predation efficiency is less disturbed by wave motion.     

Range expansion of a habitat-modifying species leads to loss of taxonomic diversity: a new and impoverished reef state

Global climate change is predicted to have major negative impacts on biodiversity, particularly if important habitat-modifying species undergo range shifts. The sea urchin Centrostephanus rodgersii (Diadematidae) has recently undergone poleward range expansion to relatively cool, macroalgal dominated rocky reefs of eastern Tasmania (southeast Australia). As in its historic environment, C. rodgersii in the extended range is now found in association with a simplified 'barrens' habitat grazed free of macroalgae. The new and important role of this habitat-modifier on reef structure and associated biodiversity was clearly demonstrated by completely removing C. rodgersii from incipient barrens patches at an eastern Tasmanian site and monitoring the macroalgal response relative to unmanipulated barrens patches. In barrens patches from which C. rodgersii was removed, there was a rapid proliferation of canopy-forming macroalgae (Ecklonia radiata and Phyllospora comosa), and within 24 months the algal community structure had converged with that of adjacent macroalgal beds where C. rodgersii grazing was absent. A notable scarcity of limpets on C. rodgersii barrens in eastern Tasmania (relative to the historic range) likely promotes rapid macroalgal recovery upon removal of the sea urchin. In the recovered macroalgal habitat, faunal composition redeveloped similar to that from adjacent intact macroalgal beds in terms of total numbers of taxa, total individuals and Shannon diversity. In contrast, the faunal community of the barrens habitat is overwhelmingly impoverished. Of 296 individual floral/faunal taxa recorded, only 72 were present within incipient barrens, 253 were present in the recovered patches, and 221 were present within intact macroalgal beds. Grazing activity of C. rodgersii results in an estimated minimum net loss of approximately 150 taxa typically associated with Tasmanian macroalgal beds in this region. Such a disproportionate effect by a single range-expanding species demonstrates that climate change may lead to unexpectedly large impacts on marine biodiversity as key habitat-modifying species undergo range modification.     

Differential survival of macroalgae to digestion by intertidal herbivore molluscs

This study evaluates the capacity of algal propagules to survive digestion by seven intertidal herbivore molluscs. Thirty individuals of Littorina peruviana, 36 of Siphonaria lessoni, 35 of Collisella ceciliana, and 25 each of C. zebrina, Chiton granosus, Fissurella crassa and F. limbata were collected at different dates between October, 1983 and April, 1984. About half of the individuals of each species were examined for gut contents and the other half was used to provide faecal pellets for culture. Diet of these grazers ranged from 8 macroalgal species in Collisella zebrina to 17 in Fissurella limbata. In total, macroalgal propagules of 56% of the 27 algal species found in these gut contents survive digestion. There was no positive correlation between diet and number of algal taxa growing in the respective faecal cultures. Survival through Littorina peruviana and Chiton granosus was low (20–30% of the algal species consumed) while it was high (75–83%) through the two species of Collisella. No significant correlation was found between frequency of algae in the gut contents and its frequency in the faecal cultures. Algal capacity to survive digestion was almost exclusively restricted to opportunistic species, especially in the Chlorophyta and Phaeophyta. It seems that survival of algal propagules through the digestive tract of generalist grazers is a rather random phenomenon.     

Grazing effects in macroalgal communities depend on timing of patch colonization

Rocky macroalgal assemblages are typically composed of patches differing in age and species composition and grazing is generally a very important modifier of such assemblages. We hypothesized that patch colonization time determines its algal community and that grazing effects depend on the colonization time and vary with depth. We created patches by placing empty substrates at two sublittoral depths over five consecutive months, manipulated grazer entry and determined the algal species composition in each patch in the next growing season. Distinct algal colonization periods resulted in different algal assemblages. Although algal communities in our study area consist mainly of opportunistic species, thus being highly dynamic, the resulting macroalgal assemblages differed in species richness, diversity, composition, and total biomass even a year after first colonization. Substrates close to the water-surface supported a higher species richness and diversity than those in the deeper littoral. The community characteristics, total density, total biomass and species richness were only slightly, if at all affected by grazing. However, individual algal species or taxa showed varying and even contrasting responses to grazing, often differently between depths and depending on colonization time. In the deeper littoral, but not close to the water surface, grazing increased the density of filamentous brown algae while reducing the green alga Cladophora glomerata. In these taxa, grazing effects were strongest in patches colonized during the early growing season. Grazing at the colonization stage had lasting consequences for the density of several individual species.     

The Response of Experimental Rocky Shore Communities to Nutrient Additions

The aim of this study was to determine whether the experimental nutrient enrichment of littoral rocky shore communities would be followed by a predicted accumulation of fast-growing opportunistic algae and a subsequent loss of perennial benthic vegetation. Inorganic nitrogen (N) and potassium (P) was added to eight concrete mesocosms inhabited by established littoral communities dominated by fucoids. The response to nutrient enrichment was followed for almost 2 1/2 years. Fast-growing opportunistic algae (periphyton and ephemeral green algae) grew significantly faster in response to nutrient enrichment, but the growth of red filamentous algae and large perennial brown algae was unaffected. However, these changes were not followed by comparable changes in the biomass and composition of the macroalgae. The biomass of opportunistic algae was stimulated only marginally by the nutrient enrichment, and perennial brown algae (fucoids) remained dominant in the mesocosm regardless of nutrient treatment level. Established rocky shore communities thus seem able to resist the effects of heavy nutrient loading. We found that the combined effects of the heavy competition for space and light imposed by canopy-forming algae, preferential grazing on opportunistic algae by herbivores, and physical disturbance, succeeded by a marked export of detached opportunistic algae, prevented the fast-growing algae from becoming dominant. However, recruitment studies showed that the opportunistic algae would become dominant when free space was available under conditions of high nutrient loading and low grazing pressure. These results show that established communities of perennial algae and associated fauna in rocky shore environments can prevent or delay the accumulation of bloom-forming opportunistic algae and that the replacement of long-lived macroalgae by opportunistic species at high nutrient loading may be a slow process. Nutrient enrichment may not, in itself, be enough to stimulate structural changes in rocky shore communities.     

Seed dispersal by waterbirds in southern Africa: comparing the roles of ectozoochory and endozoochory

相似文献   

Wind mediated dispersal of freshwater invertebrates in a rock pool metacommunity: differences in dispersal capacities and modes

Current evidence suggests regular overland transport of different freshwater invertebrates by wind, mainly over short distances. Yet, very little is known about the mechanism and scale of this process or about differences in wind dispersal dynamics and capacities among taxa and propagule types. We investigated wind dispersal of freshwater invertebrates in a cluster of temporary rock pools (spatial scale: 9,000 m²) in South Africa. Dispersing propagules and propagule bank fragments (i.e. aggregates of sediments and propagules) were intercepted during 1 month using a combination of windsocks (1.5 m above ground level) and sticky traps (ground level). The potential movement of propagule bank fragments (i.e. aggregates of propagules and sediments) was also simulated by tracking inter-pool movements of differently sized artificial substrate fragments similar to dry propagule bank fragments. We detected differences in the composition of dispersing communities intercepted at different altitudes (ground level and at 1.5 m). Comparison of dispersal distance distributions also revealed significant differences among taxa. Overall, larger propagule types (e.g. adult ostracods and oribatid mites) dominantly travelled near ground level while small resting eggs and cryptobiotic life stages of copepods were most frequently collected at higher altitudes (1.5 m) and dispersed over the longest distances. Finally, not only dispersal of single propagules but also ground level transport of propagule bank fragments was shown to contribute to local dispersal dynamics in temporary aquatic habitats.     

Algal structure of the littoral epilithon in lentic water bodies at Hope Bay, Antarctic Peninsula

We studied the composition and density of the algae of the littoral epilithon and the taxa turnover in nine lentic water bodies at Hope Bay, Antarctic Peninsula, during summer 2002. At each site we measured the main physical and chemical variables, and took epilithic samples for the algal analysis. Two composed samples (one for qualitative and one for quantitative analysis) were taken randomly about 1 m away from the shore-line of each sampling site. The morphology of the algal mats could be included in the ‘moat and pond type’ in which Cyanobacteria are well represented. A total of 69 algal taxa were recorded in the whole study area. Chlorophyceae showed the highest taxa richness (33%), followed by Cyanobacteria (29%), Bacillariophyceae (26%), Chrysophyceae (10%), and Tribophyceae (2%). As a result of the redundancy analysis performed, we found that the number, composition, and density of taxa in the water bodies at Hope Bay were strongly influenced by nitrate and suspended solids concentrations and by the distance from the sea. The geographical trend in taxa turnover within Hope Bay was explained by elements from the combination of the metapopulation dynamics and the continuum theories, which are related to dispersal limitation and environmental heterogeneity, respectively.     

The roles of environmental conditions and spatial factors in controlling stream macroalgal communities

In the last three decades, several studies have suggested that the structure of stream macroalgal communities is shaped by local environmental variables, but some recent papers have shown that the relevance of the environment on these communities may be overestimated. Using Partial Redundancy Analysis (pRDA), we analyzed macroalgal communities (considering all macroalgae and Phyla Chlorophyta, Cyanobacteria, and Rhodophyta individually) from 105 streams in southern Brazil to test the hypothesis that the relative contributions of the environment and space on the taxonomic composition of these communities is mainly determined by the biological traits and dispersal mechanisms typical for each group. The pRDA showed that the taxonomic composition of the entire community and green algae were explained by both space and environment, whereas for cyanobacteria, only the environment was significant, and for red algae, only space was significant. These divergences in the relative contribution among algal phyla were consistent with our initial hypothesis and can be ascribed to the differences in the ecological features of each group. Our results also support the idea that the community structure of organisms with low dispersal is influenced more significantly by spatial processes, whereas for organisms with high dispersal the local environmental variables are more influential.     

The Recent Terebratulina Community in the rocky subtidal zone of the Bay of Fundy, Canada

A distinctive rocky subtidal benthic community (the Terebratulina Community) is described from the Bay of Fundy, Canada. It is shown to consist of three closely intcrspersed sub-communities: a cavity sub-community, characterised by chitons, coelenterates, brachiopods, bryozoans, chordates and annelids; a rock-face subcommunity, similar in composition but enriched in brachiopods and sponges; and an upper-surface subcommunity, dominated by algae, chitons, bivalves and echinoids.
Comparison with recently described Jurassic, Cretaceous and Recent (Mediterranean) hard-ground communities suggests a relative constancy in composition, in terms of higher taxa, since the Mesozoic. Development of these (sub-)communities occurs wherever crevice or cavity systems on hard substrates lead to microenvironments differentiated mainly on the basis of water energy and light.
The composition and trophic structure of the community and the life habits of Terebratulina septentrionalis (Couthouy) are related to aspects of the environment. Marked differences in composition between the living cavity sub-community and the death assemblage in the sediments are shown to be due to differential preservation, fragmentation, population dynamics and limited local transportation.     

Effects of sedimentation on macroalgae: species-specific responses are related to reproductive traits

Although increases in sedimentation have been proposed to interfere with benthic communities in many coastal areas worldwide, few experimental studies have investigated the effect of sedimentation on community composition and assessed species-specific responses. In a field experiment on a rocky shore on the Swedish east coast (northern Baltic Proper) we confirmed the hypotheses that ambient sedimentation influences macroalgal abundance and community composition, and that species-specific responses to sedimentation correlate with reproductive strategies. We followed the establishment and development of macroalgal vegetation on artificial substrates at 8-m and 15-m depth for 4.5 years while manipulating the depositional environment by regularly removing accumulated sediment. Sediment removal significantly favoured macroalgal development and vegetation cover. Responses of macroalgal species to the sediment treatments were clearly species-specific; for example, the ephemeral green algae (Cladophora glomerata and Enteromorpha spp.) were highly tolerant to sedimentation while belt-forming perennial brown algae (Fucus vesiculosus and Sphacelaria arctica) were not. Accordingly, multivariate analyses (redundancy analysis) showed that variance in species abundances were significantly correlated to sediment conditions. The effect of sediment removal was higher at 15-m than at 8-m depth and some species distributions seemed limited in depth by the present sediment load (e.g. F. vesiculosus). Vegetative propagation was common in the study area and many species mainly depended on dispersal by fragmentation. Generally, species with an extended reproductive period, either by long continuous spore release (C. glomerata and Enteromorpha spp.) or vegetative dispersal by fragmentation (e.g. Furcellaria lumbricalis and Polysiphonia fucoides), were most tolerant to sedimentation. This paper demonstrates long-term effects of sediment deposition on the development of a macroalgal community over several growing seasons. The results indicate that variation in sediment loads is an important constraint for species local distributions and abundances, and affects the composition of sublittoral rocky-shore macroalgal communities.     

Seasonality and distribution of epilithic diatoms,macroalgae and macrophytes in a spring-fed stream system in Ontario,Canada

A study of the epilithic diatom, macroalgal and macrophyte communities from a spring-fed stream in Ontario, Canada was undertaken from September 1996 to July 1997. The relative abundance of the epilithic diatom flora, percent cover of macroalgal and macrophyte taxa, and several physical and chemical stream conditions were monitored along a 20-m stretch at each of four sites, approximately every 2 months. Several stream conditions were relatively constant over the sampling period (pH, maximum width and maximum depth), while others exhibited a distinct seasonal pattern (water temperature, specific conductance and daylength) and some fluctuated strongly with no discernable seasonal pattern (turbidity, current velocity). A total of 124 taxa were identified from the four sites, including 79 epilithic diatoms, three macroalgal diatom species (large gelatinous masses), one cyanobacterium, two red algae, eight green algae, one chrysophyte alga, one tribophyte alga, three mosses, three horsetails and 23 angiosperm taxa. Species richness was positively correlated to stream channel maximum width and depth, indicating that the total number of species tends to increase in a downstream direction. Distribution of several diatom and macroalgal species was significantly correlated to stream conditions (e.g. Gomphonema parvulum and Phormidium subfuscum with current velocity); however, the vast majority of species did not display seasonal variation in abundance that could be explained by changes in stream conditions. Many of the taxa identified from Blue Springs Creek are common elsewhere in North America.     

Passive external transport of freshwater invertebrates by elephant and other mud‐wallowing mammals in an African savannah habitat

1. Recent findings hint at the potential importance of mammals affecting the spatial dynamics of aquatic organisms in areas where mammals live in close association with water. Perhaps the most iconic example of such an environment is the African savannah. 2. We investigated dispersal patterns of freshwater organisms among a set of temporary ponds in SE Zimbabwe to test the hypothesis that large mammals, and particularly African elephants (Loxodonta africana), can be important vectors of aquatic organisms. Dispersal kernels were reconstructed by hatching mud collected from ‘rubbing’ trees located at increasing distances from a set of isolated ponds. To assess the relative importance of other mammalian vectors, the vertical distribution of mud on rubbing trees was mapped and related to the body size of candidate vector species. 3. Laboratory hatching of mud samples revealed large numbers of propagules of 22 invertebrate taxa as well as some aquatic macrophytes. Dispersing communities reflected source communities and diverged with increasing distance from the source. Both dispersal rates and richness of transported taxa decreased significantly with dispersal distance. No indications for differences in dispersal capacity among propagule types were detected. Instead, common propagules were more likely to travel greater distances. Most mud was attached to trees at heights >1.5 m, implicating elephants as the dominant vector. Vertical distributions of tree mud, however, also revealed clustering at heights up to 50 cm and 90–120 cm corresponding to the height of warthog, rhinoceros and buffalo, respectively. Finally, variation in the vertical distribution of mud on trees in combination with differences in vector vagility suggests that local differences in vector species composition may affect passive dispersal dynamics of aquatic organisms. 4. Based on vagility and vector load, mud‐wallowing mammals emerge as highly effective vectors that, in some areas, may be more important in transporting aquatic organisms than traditionally recognised vectors such as waterbirds. Since most large‐ and medium‐sized mammals currently have restricted geographic distributions, it is likely that mammal‐mediated dispersal was more important in the past.     

Airborne dispersal of antarctic terrestrial algae and cyanobacteria

The dispersal of algae and cyanobacteria at three Antarctic fellfield sites was investigated using microscopic and culture analysis of samples from active and passive air samplers Intersite variation in the mean number of large algal propagules (>5 μm diameter) sampled was dependent on the niche space available for algal growth and the degree to which soil was exposed to desiccating influences, these factors could be related to the degree of maturity of the sue The numbers of large algal propagules were lowest at sites from which permanent snow cover had recently disappeared and highest at sites with developed soil circles but poorly developed moss and lichen flora Mature sites with diverse and developed moss and lichen flora produced intermediate numbers of algal propagules Propagules of multicellular algae, cyanobacteria and large-celled unicellular algae were found in the air at the end of the growing season of the respective algal groups as the soil surface dried This was the case for Prasiola crispa, Pmnularia borealis , snow algae and filamentous chlorophytes and cyanobacteria Dispersal of unicellular chlorophytes was greatest during the summer period and at sites with developed secondary flora, but also occurred at other sites and in association with small thaw events during winter Cultures were obtained from samples collected whilst an air mass that had originated in South America, deposited material on Signy Island This suggests that algal propagules have the ability to survive long-distance transport and potentially provide mocula for colonization of Antarctica as regional warming continues to expose fresh habitats     

Cascading Effects of Ocean Acidification in a Rocky Subtidal Community

Temperate marine rocky habitats may be alternatively characterized by well vegetated macroalgal assemblages or barren grounds, as a consequence of direct and indirect human impacts (e.g. overfishing) and grazing pressure by herbivorous organisms. In future scenarios of ocean acidification, calcifying organisms are expected to be less competitive: among these two key elements of the rocky subtidal food web, coralline algae and sea urchins. In order to highlight how the effects of increased pCO₂ on individual calcifying species will be exacerbated by interactions with other trophic levels, we performed an experiment simultaneously testing ocean acidification effects on primary producers (calcifying and non-calcifying algae) and their grazers (sea urchins). Artificial communities, composed by juveniles of the sea urchin Paracentrotus lividus and calcifying (Corallina elongata) and non-calcifying (Cystoseira amentacea var stricta, Dictyota dichotoma) macroalgae, were subjected to pCO₂ levels of 390, 550, 750 and 1000 µatm in the laboratory. Our study highlighted a direct pCO₂ effect on coralline algae and on sea urchin defense from predation (test robustness). There was no direct effect on the non-calcifying macroalgae. More interestingly, we highlighted diet-mediated effects on test robustness and on the Aristotle''s lantern size. In a future scenario of ocean acidification a decrease of sea urchins'' density is expected, due to lower defense from predation, as a direct consequence of pH decrease, and to a reduced availability of calcifying macroalgae, important component of urchins'' diet. The effects of ocean acidification may therefore be contrasting on well vegetated macroalgal assemblages and barren grounds: in the absence of other human impacts, a decrease of biodiversity can be predicted in vegetated macroalgal assemblages, whereas a lower density of sea urchin could help the recovery of shallow subtidal rocky areas affected by overfishing from barren grounds to assemblages dominated by fleshy macroalgae.     

Response of macroalgae and macroinvertebrates to anthropogenic disturbance gradients in rocky shores

The compliance of macroalgal and macroinvertebrate assemblages to anthropogenic disturbance gradients (e.g., nutrient enrichment) was investigated at intertidal rocky shores. Macroalgae and macroinvertebrates presented parallel behavior, both showing shifts in the communities’ structural variation along the gradients, in which an higher number of opportunistic species (and higher abundances) were found in more stressful sites (close to the disturbance source), in contrast to less disturbed sites (far from the disturbance source), which showed higher presence of more sensitive species (and higher abundance of several of them).The macroinvertebrate abundance and taxonomic composition, which are parameters required by the Water Framework Directive (WFD) to be included in tools for the ecological quality status assessment, responded to the disturbance gradient. Results suggest that the macroinvertebrate biological element might be considered an indicator of disturbance in intertidal rocky shores as good as the macroalgae, and therefore the development of a specific methodology based solely on benthic macroinvertebrates of rocky shores, presently a gap in the ecological quality status assessment for the WFD, seems feasible.     

Tracing the origin of green macroalgal blooms based on the large scale spatio-temporal distribution of Ulva microscopic propagules and settled mature Ulva vegetative thalli in coastal regions of the Yellow Sea,China

From 2008 to 2016, massive floating green macroalgal blooms occurred annually during the summer months in the Yellow Sea. The original source of these blooms was traced based on the spatio-temporal distribution and species composition of Ulva microscopic propagules and settled Ulva vegetative thalli monthly from December 2012 to May 2013 in the Yellow Sea. High quantities of Ulva microscopic propagules in both the water column and sediments were found in the Pyropia aquaculture area along the Jiangsu coast before a green macroalgal bloom appeared in the Yellow Sea. The abundance of Ulva microscopic propagules was significantly lower in outer areas compared to in Pyropia aquaculture areas. A molecular phylogenetic analysis suggested that Ulva prolifera microscopic propagules were the dominant microscopic propagules present during the study period. The extremely low biomass of settled Ulva vegetative thalli along the coast indicated that somatic cells of settled Ulva vegetative thalli did not provide a propagule bank for the green macroalgal blooms in the Yellow Sea. The results of this study provide further supporting evidence that the floating green macroalgal blooms originate from green macroalgae attached to Pyropia aquaculture rafts along the Jiangsu coastline of the southern Yellow Sea.     

Propagule banks, herbivory and nutrient supply control population development and dominance patterns in macroalgal blooms

Destructive macroalgal mass blooms threaten estuarine and coastal ecosystems worldwide. We asked which factors regulate macroalgal bloom intensity, distribution and species composition. In field experiments in the Baltic Sea, we analyzed the relative effects of nutrients, herbivores and algal propagule banks on population development and dominance patterns in two co-occurring bloom-forming macroalgae, Enteromorpha intestinalis and Pilayella littoralis . Both species were highly affected by the combined effects of a propagule bank, herbivory and nutrients. The magnitude of effects varied with season. The propagule bank was an important overwintering mechanism for both algae, and allowed for recruitment two months earlier than recruitment via freshly dispersed propagules. This provided a seasonal escape from intense herbivory and nutrient limitation later in the year. Favored by massive recruitment from the propagule bank, Enteromorpha was the superior space occupier in early spring, thereby reducing recruitment of Pilayella . Elimination of the propagule bank and recruitment via freshly dispersed propagules favored Pilayella . Strong and selective herbivory on Enteromorpha supported Pilayella in the presence, but not in the absence of the propagule bank. Nutrient enrichment in summer counteracted herbivore pressure on Enteromorpha , thereby negatively affecting Pilayella . Herbivore and nutrient effects were more pronounced for early life stages than adult algae. These results show that recruitment processes and forces affecting early life stages at the beginning of the vegetation period determine development and dominance patterns of macroalgal blooms. Herbivores naturally suppress blooms but increasing nutrient enrichment can override this important control mechanism. The propagule bank plays a previously unrecognized role for population and community dynamics.