Assemblages Of Macroalgae Research Articles

Species-specific biomass drives macroalgal benthic primary production on temperate rocky reefs

Temperate rocky reefs dominated by the giant kelp, Macrocystis pyrifera, support diverse assemblages of benthic macroalgae that provide a suite of ecosystem services, including high rates of primary production in aquatic ecosystems. These forests and the benthic macroalgae that inhabit them are facing both short-term losses and long-term declines throughout much of their range in the eastern Pacific Ocean. Here, we quantified patterns of benthic macroalgal biomass and irradiance on rocky reefs that had intact kelp forests and nearby reefs where the benthic macroalgae had been lost due to deforestation at three sites along the California, USA and Baja California, MEX coasts during the springs and summers of 2017 and 2018. We then modeled how the loss of macroalgae from these reefs impacted net benthic productivity using species-specific, mass-dependent rates of photosynthesis and respiration that we measured in the laboratory. Our results show that the macroalgal assemblages at these sites were dominated by a few species of stipitate kelps and fleshy red algae whose relative abundances were spatially and temporally variable, and which exhibited variable rates of photosynthesis and respiration. Together, our model estimates that the dominant macroalgae on these reefs contribute 15 to 4,300 mg C m−2 d−1 to net benthic primary production, and that this is driven primarily by a few dominant taxa that have large biomasses and high rates of photosynthesis and / or respiration. Consequently, we propose that the loss of these macroalgae results in the loss of an important contribution to primary production and overall ecosystem function. Key words: benthic production; ecosystem function; kelp forest; macroalgae; primary production

Regional-scale variability in the response of benthic macroinvertebrate assemblages to a marine heatwave

Extreme climatic events are predicted to increase in severity as a consequence of anthropogenic climate change. In marine ecosystems, the importance of marine heatwaves (MHWs)—discrete periods of anomalously high sea temperatures—is gaining recognition. In 2011, the highest-magnitude MHW ever recorded impacted the west coast of Australia (southeast Indian Ocean). The MHW was associated with widespread mortality of habitat-forming species, including corals and kelps, and structural changes in assemblages of macroalgae and fish. However, the responses of benthic macroinvertebrate assemblages have not yet been fully documented. Here, we resurveyed 2 subtidal habitat types (reef ‘flats’ and ‘slopes’) at 4 locations (spanning >800 km of coastline and >6° of latitude) during the period 1999-2015 to examine the effects of the 2011 MHW on herbivorous macroinvertebrates (i.e. sea urchins, gastropod molluscs). Responses to the MHW varied with latitude; at our warmest study location, abundances were severely depleted, whereas no effects were detected at the coolest location. Across the entire study region, subtle but significant shifts in assemblage structure were observed due to decreased abundances of more southerly-distributed species (i.e. ‘cool’ affinity) and increased abundances of several more northerly-distributed species (i.e. ‘warm’ affinity). The 2011 MHW has had profound effects on the marine biota off the west coast of Australia, across multiple trophic levels and taxonomic groups. Here, as in many other regions, contemporary warming events are superimposed onto gradual warming trends, increasing the likelihood of abrupt changes in ecosystem structure and functioning.

Biomass and Habitat Characteristics of Epiphytic Macroalgae in the Sibuti Mangroves, Sarawak, Malaysia.

Mangroves support diverse macroalgal assemblages as epibionts on their roots and tree trunks. These algae provide nutrients to the primary consumers in the aquatic food web and have been reported to be substantial contributors to marine ecosystems. The species diversity, biomass, and habitat characteristics of mangrove macroalgae were investigated at three stations in the Sibuti mangrove estuary, Sarawak, Malaysia, from November 2012 to October 2013. Three groups of macroalgae were recorded and were found to be growing on mangrove prop roots, namely Rhodophyta (Caloglossa ogasawaraensis, Caloglossa adhaerens, Caloglossa stipitata, Bostrychia anomala, and Hypnea sp.), Chlorophyta (Chaetomorpha minima and Chaetomorpha sp.), and Phaeophyta (Dictyota sp.). The biomass of macroalgae was not influenced (p>0.05) by the season in this mangrove forest habitat. The macroalgal species Hypnea sp. contributed the highest biomass at both Station 1 (210.56 mg/cm2) and Station 2 (141.72 mg/cm2), while the highest biomass was contributed by B. anomala (185.89 mg/cm2) at Station 3. This study shows that the species distribution and assemblages of mangrove macroalgae were influenced by environmental parameters such as water nutrients, dissolved solids, and salinity in the estuarine mangrove habitats of Sibuti, Sarawak.

Variation in the composition of corals, fishes, sponges, echinoderms, ascidians, molluscs, foraminifera and macroalgae across a pronounced in-to-offshore environmental gradient in the Jakarta Bay–Thousand Islands coral reef complex

Substrate cover, water quality parameters and assemblages of corals, fishes, sponges, echinoderms, ascidians, molluscs, benthic foraminifera and macroalgae were sampled across a pronounced environmental gradient in the Jakarta Bay–Thousand Islands reef complex. Inshore sites mainly consisted of sand, rubble and turf algae with elevated temperature, dissolved oxygen, pH and chlorophyll concentrations and depauperate assemblages of all taxa. Live coral cover was very low inshore and mainly consisted of sparse massive coral heads and a few encrusting species. Faunal assemblages were more speciose and compositionally distinct mid- and offshore compared to inshore. There were, however, small-scale differences among taxa. Certain midshore sites, for example, housed assemblages resembling those typical of the inshore environment but this differed depending on the taxon. Substrate, water quality and spatial variables together explained from 31% (molluscs) to 72% (foraminifera) of the variation in composition. In general, satellite-derived parameters outperformed locally measured parameters.

Cover

Taxon‐specific measurements of biomass provide reliable estimates of annual net primary production by entire assemblages of macroalgae in giant kelp forests off Santa Barbara, California, USA. Photo by Ron McPeak. [Vol. 49, No. 2, pp. 248–257]

Patterns and controls of the dynamics of net primary production by understory macroalgal assemblages in giant kelp forests.

Macroalgae are important primary producers in many subtidal habitats, yet little information exists on the temporal and spatial dynamics of net primary production (NPP) by entire subtidal assemblages. This knowledge gap reflects the logistical challenges in measuring NPP of diverse macroalgal assemblages in shallow marine habitats. Here, we couple a simple primary production model with nondestructive estimates of taxon-specific biomass on subtidal reefs off Santa Barbara, California to produce a 4-year time series of net primary production by intact assemblages of understory macroalgae in giant kelp forests off Santa Barbara, California, USA. Daily bottom irradiance varied significantly throughout the year, and algal assemblages were on average exposed to saturating irradiance for only 1.3-4.5h per day, depending on the time of year. Despite these variable light-limiting conditions, biomass rather than irradiance explained the vast majority of variation observed in daily NPP at all times of the year. Measurements of peak biomass in spring and summer proved to be good predictors of NPP for the entire year, explaining as much as 76% of the observed variation. In contrast, bottom irradiance was a poor predictor of NPP, explaining <10% of the variation in NPP when analyzed seasonally and ~2% when evaluated annually. Our finding that annual NPP by macroalgal assemblages can be predicted from a single, nondestructive measurement of biomass should prove useful for developing time series data that are necessary to evaluate natural and anthropogenic changes in NPP by one of the world's most productive ecosystems.

Concentration boundary layers around complex assemblages of macroalgae: Implications for the effects of ocean acidification on understory coralline algae

Metabolic processes have the potential to modulate the effects of ocean acidification (OA) in nearshore macroalgal beds. We investigated whether natural mixed assemblages of the articulate coralline macroalga Arthrocardia corymbosa and understory crustose coralline algae (CCA) altered pH and O2 concentrations within and immediately above their canopies. In a unidirectional flume, we tested the effect of water velocity (0–0.1 m s−1), bulk seawater pH (ambient pH 8.05, and pH 7.65), and irradiance (photosynthetically saturating light and darkness) on pH and O2 concentration gradients, and the derived concentration boundary layer (CBL) thickness. At bulk seawater pH 7.65 and slow velocities (0 and 0.015 m s−1), pH at the CCA surface increased to 7.90–8.00 in the light. Although these manipulations were short term, this indicates a potential daytime buffering capacity that could alleviate the effects of OA. Photosynthetic activity also increased O2 concentrations at the surface of the CCA. However, this moderating capacity was flow dependent; the CBL thickness decreased from an average of 26.8 mm from the CCA surface at 0.015 m s−1 to 4.1 mm at 0.04 m s−1. The reverse trends occurred in the dark, with respiration causing pH and O2 concentrations to decrease at the CCA surface. At all flow velocities the CBL thicknesses (up to 68 mm) were much greater than those previously published, indicating that the presence of canopies can alter the CBL substantially. In situ, the height of macroalgal canopies can be an order of magnitude larger than those used here, indicating that the degree of buffering to OA will be context dependent.

Extreme spatial variability in sessile assemblage development in subtidal habitats off southwest Australia (southeast Indian Ocean)

The development of marine benthic communities is strongly influenced by patterns of settlement, recruitment and survival, which may vary across multiple spatial scales in concordance with the scale-dependent processes that drive them. The temperate subtidal reefs off southwest Australia support highly diverse assemblages of macroalgae and sessile invertebrates, yet little is known about spatial variability in the structure of developing assemblages compared with established assemblages. Here, settlement panel arrays were deployed adjacent to subtidal rocky reefs, in 13–15m depth, at 3 locations spanning 400km of temperate coastline in Western Australia. Panel assemblages were allowed to develop for ~14months before they were harvested. Variability in ecological pattern was analyzed at 4 spatial scales, spanning centimeters to 100s of kilometers. The structure of sessile assemblages was vastly different between the 3 locations, in that one location (Geographe Bay) supported an impoverished assemblage comprising a single macrofaunal species whereas assemblages at the other two locations (Jurien Bay and Marmion Lagoon) supported fairly rich assemblages of macroalgae and sessile invertebrates. Multivariate assemblage structure, total richness and total cover varied significantly between the locations, although variability at the smallest spatial scale (centimeters) was consistently pronounced. Variability patterns for key taxa were less consistent across spatial scales. While explanations for the extreme between-location variability remain unclear, there was some evidence to suggest that herbivory by demersal fish may inhibit assemblage development at Geographe Bay, although local hydrodynamic factors (i.e. relatively lower water movement and influence of the dominant regional-scale oceanic current) could also be important.

Chemical and Physical Environmental Conditions Underneath Mat- and Canopy-Forming Macroalgae, and Their Effects on Understorey Corals

Disturbed coral reefs are often dominated by dense mat- or canopy-forming assemblages of macroalgae. This study investigated how such dense macroalgal assemblages change the chemical and physical microenvironment for understorey corals, and how the altered environmental conditions affect the physiological performance of corals. Field measurements were conducted on macroalgal-dominated inshore reefs in the Great Barrier Reef in quadrats with macroalgal biomass ranging from 235 to 1029 g DW m−2 dry weight. Underneath mat-forming assemblages, the mean concentration of dissolved oxygen was reduced by 26% and irradiance by 96% compared with conditions above the mat, while concentrations of dissolved organic carbon and soluble reactive phosphorous increased by 26% and 267%, respectively. The difference was significant but less pronounced under canopy-forming assemblages. Dissolved oxygen declined and dissolved inorganic carbon and alkalinity increased with increasing algal biomass underneath mat-forming but not under canopy-forming assemblages. The responses of corals to conditions similar to those found underneath algal assemblages were investigated in an aquarium experiment. Coral nubbins of the species Acropora millepora showed reduced photosynthetic yields and increased RNA/DNA ratios when exposed to conditions simulating those underneath assemblages (pre-incubating seawater with macroalgae, and shading). The magnitude of these stress responses increased with increasing proportion of pre-incubated algal water. Our study shows that mat-forming and, to a lesser extent, canopy-forming macroalgal assemblages alter the physical and chemical microenvironment sufficiently to directly and detrimentally affect the metabolism of corals, potentially impeding reef recovery from algal to coral-dominated states after disturbance. Macroalgal dominance on coral reefs therefore simultaneously represents a consequence and cause of coral reef degradation.

Evaluation of algal regulation by herbivorous fishes on Caribbean coral reefs

The role of herbivorous fishes in maintaining low macroalgal cover was evaluated on coral reefs on several reef sites from Guadeloupe, either protected or not. Grazing by herbivorous fishes was assessed on different algal facies using fish-bite counts. Algal consumption by fish was estimated as well as algal production. Bite counts revealed that herbivorous fishes feed preferentially on algal turf and avoid brown macroalgae. The algal consumption varied between 0.4 and 2.8 g m−2 days−1 and was higher inside marine protected areas than outside. Comparison with algal production revealed that herbivorous fishes did not succeed in regulating algal growth. The insufficient number of grazers may lead to the dominance of stable assemblages of macroalgae on coral reefs, preventing the recovery of reef into previous coral-dominated ecosystems.

Density derived estimates of standing crop and net primary production in the giant kelp Macrocystis pyrifera

Assemblages of macroalgae are believe to be among the most productive ecosystems in the world, yet difficulties in obtaining direct estimates of biomass and primary production have led to few macroalgal data sets from which the consequences of long-term change can be assessed. We evaluated the validity of using two easily measured population variables (frond density and plant density) to estimate the more difficult to measure variables of standing crop and net primary production (NPP) in the giant kelp Macrocystis pyrifera off southern California. Standing crop was much more strongly correlated to frond density than to plant density. Frond density data collected in summer were particularly useful for estimating annual NPP, explaining nearly 80% of the variation in the NPP from year to year. Data on frond densities also provided a relatively good estimate of seasonal NPP for the season that the data were collected. In contrast, estimates of seasonal and annual NPP derived from plant density data were less reliable. These results indicate that data on frond density collected at the proper time of year can make assessments of NPP by giant kelp more tractable. They also suggest that other easily measured variables that are strongly correlated with standing crop, such as surface canopy area, might serve as similarly useful proxies of NPP.

BIODIVERSITY MEDIATES PRODUCTIVITY THROUGH DIFFERENT MECHANISMS AT ADJACENT TROPHIC LEVELS

Biodiversity may enhance productivity either because diverse communities more often contain productive species (selection effects) or because they show greater complementarity in resource use. Our understanding of how these effects influence community production comes almost entirely from studies of plants. To test whether previous results apply to higher trophic levels, we first used simulations to derive expected contributions of selection and complementarity to production in competitive assemblages defined by either neutral interactions, dominance, or a trade-off between growth and competitive ability. The three types of simulated assemblages exhibited distinct interaction signatures when diversity effects were partitioned into selection and complementarity components. We then compared these signatures to those of experimental marine communities. Diversity influenced production in fundamentally different ways in assemblages of macroalgae, characterized by growth-competition trade-offs, vs. in herbivores, characterized by dominance. Forecasting the effects of changing biodiversity in multitrophic ecosystems will require recognizing that the mechanism by which diversity influences functioning can vary among trophic levels in the same food web.

Stasis or kinesis? Hidden dynamics of a rocky intertidal macrophyte mosaic revealed by a spatially explicit approach

Macrophyte mosaics, or tile-like assemblages of turfy marine macroalgae and surfgrass ( Phyllospadix scouleri), are persistent and highly diverse along the central Oregon coast. To test the hypothesis that spatial pattern and species abundances are relatively invariant in this system, we studied community structure, disturbance, and species interactions from 1985 to 1990. Abundances and disturbance in permanently marked plots at each of five sites spanning wave-exposed to wave-protected areas were monitored photographically each year. The analysis was spatially explicit, incorporated position effects, and allowed determination of species displacements. To interpret the potential influence of substratum on disturbance, we quantified rock hardness and sediment depth at each site. Field experiments tested the role of grazers and spatial interactions on maintenance of between-patch boundaries. Mosaic dynamics varied with wave exposure. At wave-exposed and wave-protected sites, average patterns of abundance and assemblage structure were relatively constant through time, but analysis of transition probabilities showed high rates of change among mosaic elements at wave-exposed sites and low rates of change at wave-protected sites. At wave-exposed sites, most changes involved Phyllospadix displacing neighboring macroalgal turfs but rarely the reverse. At all wave-exposures, surfgrass was the most frequently disturbed mosaic element. Disturbed areas were quickly colonized by macroalgae. At wave-exposed sites, disturbances were closed by regrowth of surfgrass. Disturbance rates were similar across wave-exposures, with wave forces causing most loss at wave-exposed sites and a combination of substratum failure and sediment burial causing most loss at wave-protected sites. At wave-exposed sites, disturbances tended to be larger (436.7 vs. 278.6 cm 2) but less numerous (228 vs. 484 total disturbances) than at wave-protected sites. At wave-exposed sites, surfgrass overgrew all other species except the kelp Lessoniopsis littoralis, which was competitively equivalent to surfgrass. Grazing had no effect on spatial interactions. Disturbance prevented surfgrass monocultures, and with variable dispersal and patchy recruitment, maintained mosaic structure. At wave-protected sites, standoffs were the usual outcome of interactions, and patchiness resulted primarily from colonization of disturbances and subsequent succession. Like mussels, Phyllospadix are simultaneously dominant competitors, the most disturbance-susceptible species, and poor colonizers. These features are shared by theoretical models exploring the processes underlying spatially structured assemblages, and may characterize spatially structured systems in general.

Importance of canopy structure on photosynthesis in single‐ and multi‐species assemblages of marine macroalgae

Plant communities utilize available irradiance with different efficiency depending not only on their photosynthetic characteristics but also on the canopy structure and density. The importance of canopy structure are well studied in terrestrial plant communities but poorly studied in aquatic plant communities. The objective of this study was to evaluate macroalgal community photosynthesis in artificial constructed communities of one to four species with different morphologies along a range of leaf (i.e.=thallus) area densities. In a laboratory set‐up we measured net photosynthesis and dark respiration in constructed assemblages of macroalgae, excluding effects other than photosynthesis of individual tissue and distribution of photons in the canopy from influencing metabolism. We hypothezised that 1) canopy structure determines the actual rates of photosynthesis relative to the optimal rates and 2) multi‐species communities attain higher maximum photosynthetic rates than single species communities. We found that differences in canopy structure outweighed large differences in tissue photosynthesis resulting in relatively similar maximum community photosynthetic rates among the different single and multi‐species assemblages (20.1–40.5 μmol O2 m−2 s−1). Canopy structure influenced community photosynthesis both at low and high leaf area densities because it determines the ability of macroalgae to use the photosynthetic potential of their individual tissues. Due to an averaging effect the photosynthetic rate at high leaf area density was more similar among multi‐species community than among single‐species communities. Multi‐species communities had, on average, a slightly higher photosynthetic production than expected from photosynthesis of single species communities. Moreover multi‐species communities were capable of exposing new tissue to irradiance up to high densities thereby avoiding a decrease in net photosynthesis. This finding suggests that multi‐species communities may be able to maintain higher biomass per unit ground area than single‐species communities.

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.

The relative importance of nutrient enrichment and herbivory on macroalgal communities near Norman's Pond Cay, Exumas Cays, Bahamas: a “natural” enrichment experiment

The simultaneous effects of grazing and nutrient enrichment on macroalgal communities were experimentally investigated using plastic mesh enclosure/exclosure cages along a natural nutrient (DIN, SRP) gradient from the discharge of a tidal mangrove creek on the west side of Norman's Pond Cay, Exumas Cays, Bahamas. Nutrient enrichment was the only factor affecting total biomass of macroalgae whereas selective herbivory moderated species composition. Biomass ranged from >2 kg dry weight m −2 at the DIN-enriched Waterfall to <600 g dry weight m −2 in the DIN-limited Algal Halo. Grazing by the queen conch Strombus gigas and the sea hare Aplysia dactylomela significantly reduced biomass of the epiphyte Laurencia intricata, but not its host Digenea simplex at the DIN-enriched Waterfall. These two rhodophytes dominated the macroalgal community at the DIN-enriched Waterfall and inner Algal Halo, whereas the chlorophytes Microdictyon marinum and Cladophora catenata dominated the relatively DIN-depleted outer Algal Halo. Higher grazing rates by S. gigas, A. dactylomela, and juvenile parrotfish and surgeonfish at the DIN-enriched Waterfall compared to the more oligotrophic Algal Halo suggested selective grazing on higher quality (nitrogen) diets by these herbivores. A pronounced increase in the carbon/nitrogen (C:N) ratios of D. simplex and L. intricata from the Waterfall to the Algal Halo indicated increased DIN-limitation of growth in the more offshore, lower DIN macroalgal communities. In contrast, only modest increases in C:N ratios of M. marinum and C. catenata between the Waterfall and Algal Halo suggested that these species are better adapted to growth in reef environments with lower levels of DIN enrichment. Low coral cover (<2%) co-occurred with high macroalgal cover (>35%) at DIN-enriched reef sites around Norman's Pond Cay (Waterfall, Patch Reef, North Perry Reef) compared to relatively high coral cover (10–20%) and low macroalgal cover (<20%) at the lower DIN reef sites (Rainbow Reef, Lang's Reef). These results support ecological theory that bottom-up control via nutrient enrichment is a primary factor regulating overall biomass and taxonomic assemblages of macroalgae on coral reefs, whereas grazing is more important in controlling relative species composition via dietary preferences.

Symptoms of Eutrophication in Intertidal Macroalgal Assemblages of Nuevo Gulf (Patagonia, Argentina)

The city of Puerto Madryn, located within a section of Nuevo Gulf (Patagonia, Argentina) known as Nueva Bay, can be regarded as an important centre of anthropogenic impact. Intertidal assemblages of macroalgae were studied in order to test the hypothesis that the seaweed flora impacted by raw sewage would show significant differences in structure compared to that of control localities, mainly due to the increase in biomass of ephemeral and opportunist chlorophytes. Three rocky shores showing similar characteristics were selected. One of these was located near the sewage outfall of the city of Puerto Madryn. Two control shores were chosen approximately 30 km northward and southward of the latter. Four seasonal surveys were performed between June 1998 and April 1999. Biomass data of 35 macroalgal species were analysed by non-metric multidimensional scaling and the analysis of similarities permutation test. Species responsible for the differences between shores were identified by the similarity percentages routine. The macroalgal assemblage of the impacted site was significantly different from those of the two control shores throughout the year. Ulva rigida, Corallina officinalis and Ralfsia sp. were the species mostly responsible for these differences. Results indicate that the impacted shore showed signs consistent with an intermediate degree of eutrophication, particularly the increased abundance of the opportunist green alga Ulva rigida.

A comparison of spatial and temporal patterns in epiphytic macroalgal assemblages of the seagrasses Amphibolis griffithii and Posidonia coriacea

Studies of patterns in assemblages of epiphytic macroalgae growing on seagrasses have largely focussed at the scale of leaves or shoots. Few have examined patterns at larger spatial scales or the influence of host seagrass type on these patterns, despite interest in monitoring changes in epiphyte composition that can lead to the death of seagrasses. We studied spatial pat- terns in assemblages of epiphytic macroalgae on 2 seagrass species with different morphologies and longevity, Amphibolis griffithii and Posidonia coriacea, which co-occur on Success Bank, West- ern Australia. Epiphyte assemblages were described for each species at a hierarchy of spatial scales (sites separated by 10s, 100s and 1000s m) in summer and winter. At all locations and at both times, A. griffithii supported a higher species richness. A total of 91 taxa were recorded on A. griffithii and 70 on P. coriacea. Both seagrass species showed strong temporal patterns in their epiphyte macro- algal assemblages, but this was stronger in P. coriacea. For A. griffithii spatial variation was the dominant pattern. Epiphyte assemblages of the 2 seagrasses displayed striking differences in pat- terns of dissimilarity at different spatial scales. For A. griffithii, Bray-Curtis dissimilarity (D) among pairs of samples progressively increased with increasing spatial scale (D = 0.31, 0.37 and 0.47 at scales of 10s, 100s and 1000s m, respectively). For P. coriacea, the differences among samples sepa- rated by 10s m or 100s m were relatively small (0.29 and 0.32, respectively), but the differences among samples 1000s m apart were much greater (0.52). We suggest that the differences in spatial and temporal patterns between species of seagrass may reflect an interaction of timescales of seagrass longevity with timescales of algal reproductive biology and the dispersal ranges of algal propagules. Due to the high turnover of P. coriacea leaves, the assemblages present at any given time are strongly influenced by recent recruitment events. The epiphytic assemblage of long-lived A. griffithii integrates a much longer period of recruitment, so that recent events may produce comparatively smaller shifts in composition. The longevity of A. griffithii may also permit local reinforcement of existing assemblages, so that differences in assemblage structure increase with spatial scale. The high turnover of P. coriacea leaves results in less opportunity for an assemblage to reinforce its presence, so patterns at the smaller scales are comparatively weaker. Strong patterns across km are more likely to be the result of environmental gradients. Patchiness within these gradients leads to complex, many layered spatial patterns.

Assemblages of algae and invertebrates in Southern California Phyllospadix-dominated intertidal habitats

The assemblages of macroalgae and macroinvertebrates that coexist in intertidal Phyllospadix beds at seven sites in San Diego County, California, are described quantitatively. Most of the algae are attached to benthic substrates (68 of 71 species); one of the 90 recorded invertebrates is restricted to the blades of Phyllospadix species. The composite data, considered to be characteristic of beds of both P. torreyi S. Watson and P. scouleri W. Hook. at the sites sampled, are compared qualitatively with the biota in various other adjacent and nearby habitats. With the exception of four epibiotic taxa, found only on blades of the surfgrass, the organisms are not uniquely associated with habitats where Phyllospadix plants are present. No patterns of seasonal fluctuation were evident in either species abundance or occurrences. Several ubiquitous species were consistently found in larger numbers, but most were observed infrequently.