Abundance Of Macroalgae Research Articles

Seasonality in Algal Assemblages on Upwelling-influenced Coral Reefs in the Colombian Caribbean

Seasonal changes of benthic algal assemblages have been Studied mainly in temperate and sub-tropical areas. It is not clear how natural processes contribute to the seasonal dynamics of algal assemblages on coral reefs, particularly in areas influenced by relatively cold upwelling waters. To investigate the seasonality in algal assemblages we monitored the percent cover of species and algal groups over one year (1994-1995) at two rocky-coral reefs at depths of 9-12 m on the Colombian Caribbean coast (Bahia Chengue,Tayrona National Natural Park, TNNP). The presence of relatively cold waters with temperatures of 25 degreesC and salinities of 36 ppt is indicative of upwelling events, while warm waters of 28-29 degreesC and salinities of 33 ppt indicate the rainy seasons in the area. The algal assemblage changed in composition and abundance throughout the year with a bimodal cover pattern observed for macroalgae and turf algae. During the rainy seasons (May to June and October to December) the assemblage was dominated by algal turfs (up to 43 % cover) and showed low macroalgal cover (< 20 %). In contrast, during the two upwelling periods (February and July to August) it was dominated by macroalgae (up to 44 % cover). Cover of brown macroalgae Dictyota spp. (mainly D. bartayresiana) declined from A 34 % during the upwelling to only 5 % in the non-upwelling rainy seasons. Environmental data revealed a significant inverse correlation between water temperature and macroalgal cover, suggesting that increases in macroalgae are favoured by the presence of cold, upwelling events. It is likely that upwelling events bring nutrient rich waters that stimulate macroalgal growth, however, the role of nutrients regulating the abundance of macroalgae in the TNNP is yet to be investigated. The results of this study support the argument that benthic algal assemblages of coral reefs are a highly dynamic component of these ecosystems.

TO DIVE OR NOT TO DIVE: SCUBA VERSUS ROV SAMPLING OF MACROALGAE AT 30M DEPTH

Spalding, H. L.Moss Landing Marine Laboratories, 8272 Moss Landing, Rd., Moss Landing, CA 95039 USARemotely Operated Vehicles (ROVs) and enriched air Nitrox SCUBA diving have recently become available to researchers for studying the deep‐water environment. Each use a different technique for collecting macroalgal abundance data: ROVs use collections and high‐resolution digital video which can be quantified using an integrative laser and computer imagery program (high tech), while divers often count the densities of individuals and use a point contact method for sampling percent (%) cover in situ (low tech). While the types of data collected by both techniques are the same, the effects of the different sampling methods on data resolution are unknown. As part of a larger study on deep‐water macroalgae in central California, I compared the abundance of common macroalgae (% cover of macroalgal groups and individuals/m2) collected by divers and the ROV Ventana at a depth of 30m at 3 locations in central California. Generally, there were no significant differences between diver and ROV data in the % cover of coralline Rhodophyta, non‐coralline Rhodophyta, and Pleurophycus gardneri/m2. The use of a laser‐calibrated computer imagery program and an ROV with user‐controlled lighting greatly decreased lab analysis time, and a method for sampling macroalgal layers with the ROV was developed. Thus, ROVs with high‐resolution digital video and supplemental macroalgal collections can be used to quantify deep‐water algae as accurately as in situ divers, but without the limited dive time, depth limits, and physical demands of the latter.

Large-scale associations between macroalgal cover and grazer biomass on mid-depth reefs in the Caribbean

Since the 1970s, macroalgae have become considerably more abundant on many Caribbean reefs and overfishing of grazing fishes has been implicated as a contributory factor. We explored relationships between algal cover and grazers (biomass of herbivorous fishes and abundance of the sea-urchin Diadema antillarum) on mid-depth reefs (12–15 m) in 19 areas at seven locations in Jamaica, Barbados, Belize, Grand Cayman and Cuba, between April 1997 and April 1998. Diadema antillarum density was never >0.01 m–2, while herbivorous fish biomass (acanthurids and scarids ≥12 cm total length) varied from 2–5 g m–2 in Jamaica to 17.1 g m–2 in Barbados, and was strongly correlated, negatively with macroalgal cover and positively with 'cropped' substratum (sum of 'bare', turf and crustose-coralline substrata) cover. However, overfishing of herbivorous fishes alone cannot explain the widespread abundance of macroalgae, as even on lightly fished reefs, macroalgal cover was mostly >20%. Herbivorous fish populations on those reefs were apparently only able to maintain approximately 40–60% of reef substratum in cropped states, but due to low space-occupation by coral and other invertebrates, 70–90% of substratum was available to algae. The abundance of macroalgae on lightly fished reefs may therefore be a symptom of low coral cover in combination with the continuing absence of Diadema antillarum.

Effects of herbivory, nutrient levels, and introduced algae on the distribution and abundance of the invasive macroalga Dictyosphaeria cavernosa in Kaneohe Bay, Hawaii

Since the 1960s, and possibly earlier, the macroalga Dictyosphaeria cavernosa has overgrown and displaced corals on reef slopes and outer reef flats in Kaneohe Bay, Oahu. This shift in reef community composition is generally attributed to nutrient enrichment resulting from sewage discharge. Following the diversion of most of the sewage effluent in 1977–1978, it was expected that D. cavernosa growth would become nutrient-limited and its abundance would consequently decline, but the alga remains abundant in much of the bay. One explanation for its persistence is that nutrients are once again high enough to support the alga's growth. An alternative explanation is that there has been a reduction in grazing intensity in the bay. In this study we resurveyed the distribution and abundance of D. cavernosa at 120 reef slope sites originally surveyed in 1969. We conducted additional surveys to estimate the biomass of herbivores and the areal coverage of D. cavernosa and other macroalgae on reef slopes and flats. Field experiments were used to determine spatial and temporal patterns of grazing intensity on and growth rates of D. cavernosa and the introduced macroalga Acanthophora spicifera. Laboratory experiments were used to examine preferences among herbivores for some of the most abundant macroalgae on Kaneohe Bay reefs. Twenty years after sewage diversion, D. cavernosa cover on reef slopes has decreased substantially in southern Kaneohe Bay, the site of most of the historical sewage discharge. D. cavernosa cover has changed less in other regions, remaining high in the central bay and low in the north bay. D. cavernosa thalli protected by grazer exclusion cages sustained positive growth rates on reef slopes and flats throughout the bay. Reduced nutrient concentrations may have caused a reduction in D. cavernosa growth rates, and a consequent reduction in D. cavernosa abundance in the south bay shortly after sewage diversion. Measurements of grazing intensity and surveys of herbivorous fish abundance suggest that the continued abundance of D. cavernosa is the result of a reduction in grazing intensity. Reduced grazing intensity on D. cavernosa may in turn be the result of a historical reduction in herbivore biomass or the establishment of several introduced macroalgae on reef flats. The introduced species are preferred by herbivorous fishes over D. cavernosa, as indicated by preference tests. The hypothesis that reduced grazing pressure on D. cavernosa is related to the establishment of introduced species is supported by the observation that D. cavernosa cover is highest on reef slopes where the cover of preferred introduced macroalgae on the adjacent outer reef flat is also high. Conversely, D. cavernosa cover is low or zero on reef slopes where the cover of introduced macroalgae on the adjacent reef flat is low or zero

Bottom-Up and Top-Down Forces in Tide Pools: Test of a Food Chain Model in an Intertidal Community

A simple food chain model of community structure was used to evaluate the roles of bottom-up and top-down factors in a rocky intertidal community. Predictions of the model were modified to incorporate known variation in the strength of species interactions and nutrient delivery rates along a wave-exposure gradient. To test the predictions of the model, I manipulated nutrients and consumers in tide pools chiseled into mudstone benches at two sites that varied in degree of wave exposure. The pools were located in the mid-intertidal zone between ∼1 and 1.5 m above mean lower low water (MLLW), at Boiler Bay, Oregon, USA. The focal organisms were the benthic macroalgae and mobile invertebrate herbivores that dominate naturally occurring tide pools at this site. I manipulated nutrient levels and the abundance of herbivores in these tide pools in a fully factorial randomized block design replicated six times at a wave-exposed and a wave-protected site. The experiment was maintained for two years (1994–1996). The abundances of herbivores and macroalgae were monitored in the spring, summer, and fall of each year. I measured primary productivity in the tide pools during the summer. Herbivores had a negative impact on algal abundance. The total effect of herbivory, and the efficiency of herbivores per se, on algal abundance was lower at the wave-exposed site. Nutrient additions had a positive effect on algal abundance, but this effect was reduced at the wave-exposed site. Nutrients also appeared to increase algal productivity, but only where herbivore abundance was low. Algal abundance patterns were generally consistent with model predictions for bottom-up, top-down, and hydrodynamic effects. In contrast to model predictions, herbivore abundance did not respond to the nutrient treatment. The decoupling of consumers from resource dynamics is interpreted to be the result of an herbivore preference for noncalcified seaweeds with higher potential growth rates. In wave-protected pools, where nutrients were most limiting and consumers were most efficient, seaweeds with the potential to translate elevated nutrient levels into growth had no effective refuge from consumers. The difference in scale between resource patches (tide pools) and the foraging range of the dominant herbivore, Tegula funebralis, may have augmented the ability of this herbivore to virtually exclude fleshy seaweeds from wave-protected pools. Expanding the domain of applicability of food chain models requires the incorporation of consumer preferences, variation in plant growth rates, environmental gradients, and differences in the relative scales of resource patches and foraging ranges of consumers.

BOTTOM-UP AND TOP-DOWN FORCES IN TIDE POOLS: TEST OF A FOOD CHAIN MODEL IN AN INTERTIDAL COMMUNITY

A simple food chain model of community structure was used to evaluate the roles of bottom-up and top-down factors in a rocky intertidal community. Predictions of the model were modified to incorporate known variation in the strength of species interactions and nutrient delivery rates along a wave-exposure gradient. To test the predictions of the model, I manipulated nutrients and consumers in tide pools chiseled into mudstone benches at two sites that varied in degree of wave exposure. The pools were located in the mid-intertidal zone between ∼1 and 1.5 m above mean lower low water (MLLW), at Boiler Bay, Oregon, USA. The focal organisms were the benthic macroalgae and mobile invertebrate herbivores that dominate naturally occurring tide pools at this site. I manipulated nutrient levels and the abundance of herbivores in these tide pools in a fully factorial randomized block design replicated six times at a wave-exposed and a wave-protected site. The experiment was maintained for two years (1994–1996). The abundances of herbivores and macroalgae were monitored in the spring, summer, and fall of each year. I measured primary productivity in the tide pools during the summer. Herbivores had a negative impact on algal abundance. The total effect of herbivory, and the efficiency of herbivores per se, on algal abundance was lower at the wave-exposed site. Nutrient additions had a positive effect on algal abundance, but this effect was reduced at the wave-exposed site. Nutrients also appeared to increase algal productivity, but only where herbivore abundance was low. Algal abundance patterns were generally consistent with model predictions for bottom-up, top-down, and hydrodynamic effects. In contrast to model predictions, herbivore abundance did not respond to the nutrient treatment. The decoupling of consumers from resource dynamics is interpreted to be the result of an herbivore preference for noncalcified seaweeds with higher potential growth rates. In wave-protected pools, where nutrients were most limiting and consumers were most efficient, seaweeds with the potential to translate elevated nutrient levels into growth had no effective refuge from consumers. The difference in scale between resource patches (tide pools) and the foraging range of the dominant herbivore, Tegula funebralis, may have augmented the ability of this herbivore to virtually exclude fleshy seaweeds from wave-protected pools. Expanding the domain of applicability of food chain models requires the incorporation of consumer preferences, variation in plant growth rates, environmental gradients, and differences in the relative scales of resource patches and foraging ranges of consumers.

Hydrodynamics as a Major Factor Controlling the Occurrence of Green Macroalgal Blooms in a Eutrophic Estuary: A Case Study on the Influence of Precipitation and River Management

Data on the variation of physicochemical parameters, biomass and growth of green macroalgae (mostlyEnteromorpha ) collected between January 1993 and January 1997 in the Mondego Estuary (western coast of Portugal) was analysed with the aim to identify the factors that control opportunistic macroalgal abundance in the system.The annual biomass of Enteromorpha spp. is strongly dependent on the amount of fresh water that enters the system during winter and spring. In turn, the input of fresh water is regulated by precipitation and by river management practices. The optimization of the rice crops from the upstream valley depends on their water level, which determines the number of days and hours per day during which sluice gates remain open in winter and spring. River flow has significant impacts on salinity, N:P ratios, current velocities and light extinction coefficients within the system. The interaction of all these factors controls macroalgal growth and biomass loss processes.In winters and springs during which sluice gates are often closed due to water deficiency of the rice fields (dry winter and spring or dry winter followed by rainy spring), little fresh water enters the system and consequently, salinity remains high, N:P ratios around 20, light penetration increases, and current velocities fall. These conditions facilitate macroalgal fixation, enhance their growth and spring blooms occur. On the contrary, during winters and springs when fresh water is in excess of rice fields' needs (rainy winters and springs), sluice gates remain open for long periods of time. High input of fresh water to the system causes salinity and light penetration to decrease, while N:P ratios and current velocities increase. These conditions contribute both to reduced Enteromorpha growth and higher loss of macroalgal biomass from the system to the ocean.The present work shows that the inter-annual variation of macroalgal biomass in the Mondego Estuary is controlled by hydrodynamics, which in turn depends on precipitation and on river management, according with the water needs of the upstream rice crop.

Limits to grazing by herbivorous fishes and the impact of low coral cover on macroalgal abundance on a coral reef in Belize

Widespread rises in the abundance of fleshy macroalgae on Caribbean reefs within the last 20 yr have variously been attributed to eutrophication, overfishing, or the 1983-4 mass mortality of the grazing sea urchin Diadema antillarum. However, none of those factors can satisfactorily explain why macroalgae are abundant today even on lightly fished mid-depth reefs in sparsely pop- ulated areas. Here we explore another explanation for rises in macroalgal cover on such reefs, namely that they are a side effect of declines in coral cover. We suggest that grazing by herbivorous fishes can exclude macroalgae from mid-depth reefs with high cover of hard corals, but that on low- cover reefs, the amount of space occupied by algae overwhelms the ability of grazing fishes to crop it down. We simulated the effect of 10% and 25% rises in coral cover by attaching 'pseudo-corals' (PVC tiles covered in a non-toxic anti-fouling coating) to reef substratum in 5 × 5 m plots (3 groups: control, 10%T and 25%T) on a 12 m deep forereef site in front of Ambergris Caye, Belize. Within 3 mo of attaching tiles, macroalgae had declined by approximately 10% in 10%T plots and 25% in 25%T plots compared with control plots, and these new states persisted for the duration of the experiment (macro-algae averaging 38.0% in control plots, 28.7% in 10%T plots, and 13.6% in 25%T plots). Dra- matic reductions in macroalgae may have made the 25%T plots more attractive to herbivorous fishes, as, in the later stages of the experiment, herbivorous fish biomass and acanthurid feeding rate were higher in those plots than in 10%T and control plots.

Crayfish (Orconectes rusticus) Impacts on Zebra Mussel (Dreissena polymorpha) Recruitment, Other Macroinvertebrates and Algal Biomass in a Lake-outlet Stream

Zebra mussels (Dreissena polymorpha) are beginning to colonize smaller inland lakes and streams in the eastern United States. To test the hypothesis that crayfish (Orconectes rusticus) could significantly reduce the recruitment of zebra mussels in streams, we conducted a replicated (n = 7) crayfish enclosure-exclosure experiment over 15 wk to simulate conditions during the early stages of a zebra mussel invasion. Four male O. rusticus were placed in each of seven enclosures (0.2 m2) each paired with an exclosure (week 0) in a small midwestern lake-outlet stream (mean width, 20 m) before zebra mussel settlement. At week 15 zebra mussel densities were 58% lower in enclosures than in exclosures (P < 0.05). Size frequency distributions did not differ significantly between treatments (P = 0.842; size range 1–10 mm in enclosures, 1–11 mm in exclosures). Densities (no/m2) of other macroinvertebrates did not differ significantly between enclosures and exclosures. Periphyton biomass was not significantly different between treatments, but macroalgal (Chara Nitella) biomass was present in exclosures [9 ± 7 g/m2 (mean ± 95% CL)] and absent from enclosures (P < 0.001). These results suggest that streams with moderate to high densities of crayfish will experience slower invasion by zebra mussels, lower final densities of zebra mussels and lower abundance of macroalgae.

Rapid transition in the structure of a coral reef community: the effects of coral bleaching and physical disturbance.

Coral reef communities are in a state of change throughout their geographical range. Factors contributing to this change include bleaching (the loss of algal symbionts), storm damage, disease, and increasing abundance of macroalgae. An additional factor for Caribbean reefs is the aftereffects of the epizootic that reduced the abundance of the herbivorous sea urchin, Diadema antillarum. Although coral reef communities have undergone phase shifts, there are few studies that document the details of such transitions. We report the results of a 40-month study that documents changes in a Caribbean reef community affected by bleaching, hurricane damage, and an increasing abundance of macroalgae. The study site was in a relatively pristine area of the reef surrounding the island of San Salvador in the Bahamas. Ten transects were sampled every 3-9 months from November 1994 to February 1998. During this period, the corals experienced a massive bleaching event resulting in a significant decline in coral abundance. Algae, especially macroalgae, increased in abundance until they effectively dominated the substrate. The direct impact of Hurricane Lili in October 1996 did not alter the developing community structure and may have facilitated increasing algal abundance. The results of this study document the rapid transition of this reef community from one in which corals and algae were codominant to a community dominated by macroalgae. The relatively brief time period required for this transition illustrates the dynamic nature of reef communities.

Seasonal Dynamics of Macroalgal Communities of the Northern Florida Reef Tract

Coral communities worldwide are undergoing intense degradation in response to natural and human disturbances, and many reef systems have already experienced significant declines in live coral cover associated with an increase in macroalgal abundance. Here, we document the seasonal dynamics of the macroalgal communities of the Northern Florida Reef Tract, providing a baseline for long-term studies of coral-algal competition in the area.

Effects of nutrients versus herbivores on reef algae: A new method for manipulating nutrients on coral reefs

There has been much discussion and some controversy regarding the role of nutrient enrichment versus other factors, such as altered rates of herbivory, in the degradation of coral reef ecosystems. The resolution of this controversy has been hampered by the lack of manipulative field studies testing the effects of ecologically relevant levels of nutrient enrichment on coral reef communities. We present a new method for adding ecologically relevant levels of nutrients to experimental substrates on coral reefs. The method elevates nutrients for sustained periods of time (&gt;41 d without replenishment of nutrients) and allows for testing interactions of nutrients and altered levels of herbivory. Results from an offshore reef in Key Largo, Florida, show strong effects of excluding large herbivorous fishes, negligible effects of nutrient enrichment–;or effects that are opposite of predictions, and no interaction between nutrient levels and herbivory in affecting algal abundance. Patterns observed for this reef did not confirm predictions of previously proposed models that frondose macroalgal or crustose algal abundance would be enhanced with nutrient enrichment or that dominance of filamentous turfs would be greater in unenriched conditions. In contrast to previous predictions, the abundance of larger macroalgae at this site was not increased by elevating nutrients above predicted threshold response levels of 1.0 µM for total inorganic nitrogen or 0.10 µM for soluble reactive phosphate. Also conflicting with some models, filamentous, nitrogen‐fixing cyanobacteria were enhanced, rather than suppressed, by nutrient enrichment.

Macroalgae, nutrients and phase shifts on coral reefs: scientific issues and management consequences for the Great Barrier Reef

Degradation of coral reefs often involves a “phase shift” from abundant coral to abundant macroalgae. This paper critically reviews the roles of nutrient increases in such phase shifts. I conclude that nutrient overloads can contribute to reef degradation, but that they are unlikely to lead to phase shifts simply by enhancing algal growth rates and hence allowing overgrowth of corals, unless herbivory is unusually or artificially low. Concentrations of dissolved inorganic nutrients are poor indicators of reef status, and the concept of a simple threshold concentration that indicates eutrophication has little validity. I discuss the significance and consequences of these assessments for reef management, focusing on the Great Barrier Reef, and conclude with some specific recommendations, including protection of herbivorous fishes, minimisation of terrestrial runoff, and protection of coastal reefs.

Density-related reproductive trade-offs in the green sea urchin, Strongylocentrotus droebachiensis

Green sea urchin, Strongylocentrotus droebachiensis (O.F. Muller), populations are being depleted rapidly in the Gulf of Maine and there is justified concern that the potential of this free-spawner to produce larvae may be severely inhibited. We evaluated the opposing effects of different population densities on gonad development and fertilization success, using population surveys and fertilization experiments. We determined gonad indices (gonad mass/body mass) over a range of population densities (0.1 to 250 ind. m−2) at seven sites in coastal Maine, USA, sampled at two depths (5 and 15 m). At shallow sites, we found that gonad indices declined by 50% over the 1500-fold range in adult population density. At 15 m deep locations, gonad mass was consistently low and did not vary significantly with density. Patterns of macroalgal abundance suggest urchins at high density and in deeper water were food limited. Because macroalgal cover co-varies inversely with sea urchin density, we designed field experiments to determine the interaction between sea urchin density and kelp canopy on fertilization success. On square arrays we manipulated the spacing of simulated urchins, but held their numbers constant (five sperm-filled syringes interspersed with four Nitex mesh egg containers permeable to sperm). These experiments, simulating the observed range of natural density, suggested that (1) fertilization rates decreased many times faster than individual gamete production increased over the same range in density, and (2) kelp increased fertilization success at high density when eggs were within 25 cm of a sperm source, but not when spaced 1 m apart. Additional laboratory fertilization experiments at ambient temperatures (3 to 5 °C) indicated that diluted sperm were viable for 8 h. In short, to the individual the reproductive benefits of aggregating appear to outweigh the costs; and while sperm may be limiting at low population density, eggs may remain viable long enough to be fertilized by sperm from more distant males.

Disturbance and recovery of the macroflora of a seagrass ( Halodule wrightii Ascherson) meadow in the Abrolhos Marine National Park, Brazil: an experimental evaluation of anchor damage

Anchor damage due to tourist visitation is becoming increasingly intense in the Abrolhos Marine National Park, Brazil, and is probably detrimental to the biota associated with the seagrass beds. In this study the effects of anchor damage on an algal dominated seagrass ( Halodule wrightii) bed in the national park were measured and assessed. The mean size of anchor scars was 0.16 m 2, and it was estimated that 0.5% of the seagrass beds were damaged per year by boat anchoring. The short term effect of simulated anchor damage (over 4 d) was a reduction in seagrass density, in the standing stock of Laurencia obtusa (Hudson) Lamouroux growing epiphytically on Udotea flabellum (Ellis and Solander) Lamouroux, and in the total macrophyte (seagrass + algae) standing stock. The longer-term (5, 9 and 13 month) ability of the seagrass and macroalgae to recover from damage, and possible seasonal differences in recovery ability, were determined. H. wrightii reoccupied experimentally cleared 0.25 m 2 areas by vegetative elongation, and in 9 mo short shoot, rhizome and root biomass, and short shoot and rhizome densities were similar to controls. The ability of the seagrass to recover appeared not to be seasonal, and seasonality was only found in short shoot density. Recovering seagrass sent up more short shoots per length of rhizome than plants in undamaged areas. The most abundant macroalga was the rhizophytic Udotea flabellum; other algae grew on it as epiphytes or unattached. U. flabellum could recover quickly from simulated anchor damage, though other common algae ( Dictyota mertensii (Martius) Kuetz., D. cervicornis Kützing, Padina spp. and Laurencia obtusa) showed more complex patterns of recovery that were seasonally or species dependent. A perturbation such as an anchoring has an effect on the algal flora more than a year later, although most algae species and H. wrightii can recover more quickly. Notably large areas devoid of macrophytes within the seagrass bed may have been created by the fusion of anchor scars.

Feeding selectivity of the herbivorous fish Scartichthys viridis:effects on macroalgal community structure in a temperate rocky intertidal coastal zone

The ecological importance of fish herbivory was examined in a temperate rocky intertidal zone along the central Chilean coast. In this system, the blenny Scartichthys viridis is the most abundant and only herbivorous fish. We describe its diet, determine its field food selectivity (gut contents vs macroalgal field availability comparison) and report on its laboratory dietary preferences in a multiple-choice feeding experiment. Finally, we evaluate the relative effects of fish herbivory and grazing invertebrates on macroalgal abundance, distribution and diversity patterns in the field, using replicated exclusion cages to manipulate grazing intensity. S. viridis was found to be a selective grazer; its diet was dominated by 2 macroalgae: the green Ulva and the red Gelidium, representing 32.6 and 41.4% of its total gut content biomass, respectively Ulva was consumed much above its field availability while Gelidium was selected only during fall-winter. In the laboratory, S. viridis selected the green macroalgae Ulva rigida and Codium dimorphum and avoided Gelidium chilense. We suggest that Gelidium may be included in its diet due to the limited availability of Ulva in the field. Experimental exclusion of this fish from rocky surfaces resulted in increased abundance of green foliose macroalgae (U. rigida and Enteromorpha linza), and, to a much lesser extent, of the red foliose macroalgae G. chilense, Porphyra columbina, and Mazzaella Iaminarioides, and of the brown macroalgae Colpomenia phaeodactyla and Petalonia fascia, as well as an extension of the distribution of P. columbina from the high intertidal to the mid intertidal zone, where it normally does not occur. Exclusion of grazing invertebrates resulted in a decrease in foliose macroalgal species richness, but had no effect on biomass. To our knowledge, this is the first experimental demonstration of the effect of an herbivorous fish on macroalgal community structure in a temperate rocky intertidal area. The selective feeding displayed by S. viridis, its high numerical abundance, and the results of the herbivore exclusion experiment suggest its importance in maintaining the low abundance of foliose macroalgae and the high relative cover of brown and red crustose macroalgae, characteristic of many low to mid rocky intertidal areas along the coast of central Chile.

Relative Importance of Grazing and Nutrient Controls of Macroalgal Biomass in Three Temperate Shallow Estuaries

Macroalgal biomass and competitive interactions among primary producers in coastal ecosystems may be controlled by bottom-up processes such as nutrient supply and top-down processes such as grazing, as well as other environmental factors. To determine the relative importance of bottom-up and top-down processes under different nutrient loading conditions, we estimated potential amphipod and isopod grazer impact on a dominant macroalgal species in three estuaries in Waquoit Bay, Cape Cod, Massachusetts, that are subject to different nitrogen loading rates. We calculated growth increases and grazing losses in each estuary based on monthly benthic survey data of macrophyte biomass and herbivore abundance, field grazing rates of amphipods (Microdeutopus gryllotalpa andCymadusa compta) and an isopod (Idotea baltica) on the preferred and most abundant macroalga (Cladophora vagabunda) and laboratory grazing rates for the remaining species, and in situ macroalgal growth rates. As nitrogen loading rates increased, macroalgal biomass increased (3×), eelgrass (Zostera marina) was lost, and herbivore abundance decreased (1/4×). Grazing rates increased with relative size of grazer (I. baltica > C. compta > M. gryllotalpa) and, for two of the three species investigated, were faster on algae from the high-nitrogen estuary in comparison to the low-nitrogen estuary, paralleting the increased macroalgal tissue percent nitrogen with nitrogen load. Macroalgal growth rates increased (2×) with increasing nitrogen loading rate. The comparison between estimated growth increases versus losses ofC. vagabunda biomass to grazing suggested first, that grazers could lower macroalgal biomass in midsummer, but only in estuaries subject to lower nitrogen loads. Second, the impact of grazing decreased as nitrogen loading rate increased as a result of the increased macroalgal growth rates and biomass, plus the diminished abundance of grazers. This study suggests the relative impact of top-down and bottom-up controls on primary producers varies depending on rate of nitrogen loading, and specifically, that the impact of herbivory on macroalgal biomass decreases with increasing nitrogen load to estuaries.

HABITAT STRUCTURE AND REGULATION OF LOCAL SPECIES DIVERSITY IN A STONY, UPLAND STREAM

Habitat structure may regulate species diversity at local scales, with complex habitats being normally associated with greater species richness than simple ones. We employed a new method of quantifying habitat structure to examine community structure in a stone-dwelling community of mobile macroinvertebrates inhabiting a perennial, upland stream (the Steavenson River) in southeastern Australia. We distinguished between the effects of increasing habitat structure by adding similar physical elements (habitat complexity) from that produced by adding qualitatively different sources of habitat structure (habitat heterogeneity) at spatial scales relevant to the biota. We used a field experiment to ask: (1) Does variation in habitat structure at local scales (i.e., between individual stones) result in variation in species richness (S) and numbers of individuals (N), and if so, are changes in S wrought passively by changes in N, or is there evidence of local regulation of S? (2) Are macroalgae, which are a source of habitat structure for invertebrates, also affected by stone surface structure? (3) What are the effects of habitat structure on faunal composition and body sizes? We used clay bricks as substrata and manipulated three sources of habitat structure in a crossed design: large surface pits and cracks (low density/high density); small pits caused by variation in surface texture (rough/smooth); and the abundance of macroalgae (begun with algae, begun without algae). The bricks were sampled for both fauna and epilithon on days 14 and 28 of colonization, when species richness and densities of individuals were comparable to natural stream stones. Habitat structure altered faunal diversity and abundances, with the majority of common species reaching higher abundances on creviced or rough surfaces. Rough surfaces were additionally associated with shifts in overall faunal composition and markedly smaller body sizes. Each element of habitat structure (large crevices, roughness, and macroalgae) promoted both increased species richness and densities of individuals. Rarefaction indicated that changes in S were disproportionate to changes in N, which suggests that S is regulated by local processes. Overall species richness was highest on day 14 with no difference in S between simple and complex surfaces. By day 28, simple surfaces had lost taxa relative to complex surfaces, suggesting that species richness in this stream community is regulated at a local scale, even though faunal composition changes continually and is contingent upon habitat structure. Habitat structure also affected the epilithon, suggesting that sources of habitat complexity and heterogeneity are interwoven in this system. Furthermore, the epilithon response to surface structure depended on the spatial scale of habitat complexity, with more of the red, filamentous alga Audouinella hermannii being found on rough surfaces than on smooth surfaces, but less on bricks with large crevices than without. These different responses to surface structure at different spatial scales demonstrate the importance of quantifying and manipulating substrate complexity at scales that are comparable with natural surfaces.

Historical changes and inventory of macroalgae from Königshafen Bay in the northern Wadden Sea

Knowledge on the distribution, abundance and species richness of intertidal macroalgae occurring on sandy and muddy flats of the German Wadden Sea is still incomplete. We summarize published and unpublished information available on the presence of macroalgae on the tidal flats of Konigshafen Bay (island of Sylt, North Sea), one of the more extensively studied areas of the Wadden Sea. A total of 46 green algal species, 36 brown algal species and 26 red algal species has been recorded within the last 120 years on soft and hard substrata of Konigshafen Bay (disregarding species found unattached or drifting). Several of these species were only temporarily resident on the tidal flats. Today, at least 35 green, 15 brown and 12 red algal species occur within or close to Konigshafen Bay. Significant long-term changes in species abundances have occurred in all three major groups of algae: Since the late 1970s, dense green algal mats dominated byEnteromorpha flexuosa, E. radiata andE. prolifera have occurred regularly on the intertidal flats, whereas a general decrease of brown and red algae has been documented. Two red algal species,Gracilaria verrucosa and its epiphyteCallithamnion corymbosum, were conspicuous members of the macroflora until the middle of this century. Although still present in the 1980s, they have now disappeared completely. On the other hand, the brown algaSargassum muticum has begun to colonize mussel beds. The causal background of long-term changes in the macroalgal flora of Konigshafen Bay is discussed. Owing to substantial nomenclatural changes during the last 120 years, a revised species list with authors’ names and synonyms is included.

Drift macroalgal abundance in seagrass beds:investigating large-scale associations with physical and biotic attributes

The relationship between a suite of physical and biological factors and drift macroalgal abundance was examined in 12 seagrass sites spanning a distance of over 50 km within Tampa Bay, Florida, USA. Field sampling was conducted in December 1991, and drift algal abundance was determined by recording algal percent cover from 25 quadrants at each site. Additionally, drift rnacroalgae were collected from gridded plots and dry weight b ~ o ~ n a s s measured. Sediment characteristics and seagrass blomass, shoot density, blade length and percent cover at each site were also determined Likewise, pos~tion of slte within Tampa Bay was categorized by spatlal coordinates. Step-wise regression indicated that percent silt-clay and percent seagrass cover combined to explain 57 % of the vanance of dnft algal cover In seagrass beds, while percent silt-clay alone explained 35% of algal biomass variance. Neither drift macroalgal cover nor biomass demonstrated any significant relationship wlth shoot density or aboveground biomass. Over the large spatial scale of this study, our findings suggest a link between hydrodynamic regime, as reflected by sediment characteristics, and drift macroalgal abundance in seagrass beds.