Interactive effects of grazing and enrichment on diversity; conceptual implications of a rocky intertidal experiment

Understanding the ways in which consumers and productivity act and interact to yield differences in diversity is of primary conceptual and pragmatic importance in a world in which humans are simultaneously changing ecological communities and substantially altering the availability of nutrients. Here, I used macroalgal communities on rocky reefs to examine the effects of both limpet grazing and nutrient enrichment on algal diversity throughout almost two years of succession. The experimental design included three levels each of grazing and nutrients, with unglazed terracotta pots attached to the rock as replicate plots in a high intertidal limpet-macroalgal community. Grazing effects varied by year. During the first year, grazing effects were context-dependent with limpets resulting in lower species richness, especially at the highest level of limpet density. However, at this highest level of limpet density, high enrichment counteracted the negative effect of limpets such that diversity was similar to that in treatments with lower limpet densities. In the second year, grazing generally decreased richness, regardless of enrichment. The results of this experiment are partially consistent with the grazer-reversal hypothesis – grazing decreased richness in low nutrient conditions and this effect was neutralized (rather than reversed) under high enrichment. Inconsistencies with model predictions may be explained by the apparent unresponsiveness of algal productivity to experimental enrichment, the unique substrate-scraping feeding mechanisms of limpets, and potentially limited propagule supply.     

The role of light availability and herbivory on algal responses to nutrient enrichment in a riparian wetland,Alaska

We investigated how the relative availability of solar radiation in the presence or absence of grazing alters the ability of benthic algae to respond to nutrient enrichment in an Alaskan marsh. We used a factorial mesocosm experiment that included nutrient enrichment (enriched or control), grazing (grazed or ungrazed), and light (unshaded or shaded) to simulate shading by macrophytes early and late in the growing season, respectively. We found stronger effects of grazers and nutrients compared to light on benthic algal biomass and taxonomic composition. Algal biomass increased in nutrient‐enriched treatments and was reduced by grazing. Shading did not have an effect on algal biomass or taxonomic composition, but the concentration of chl a per algal biovolume increased with shading, demonstrating the ability of algae to compensate for changes in light availability. Algal taxonomic composition was more affected by grazer presence than nutrients or light. Grazer‐resistant taxa (basal filaments of Stigeoclonium) were replaced by diatoms (Nitzschia) and filamentous green algae (Ulothrix) when herbivores were removed. The interacting and opposing influences of nutrients and grazing indicate that the algal community is under dual control from the bottom‐up (nutrient limitation) and from the top‐down (consumption by herbivores), although grazers had a stronger influence on algal biomass and taxonomic composition than nutrient enrichment. Our results suggest that low light availability will not inhibit the algal response to elevated nutrient concentrations expected with ongoing climate change, but grazers rapidly consume algae following enrichment, masking the effects of elevated nutrients on algal production.     

Gastropod grazers and nutrients,but not light,interact in determining periphytic algal diversity

The potential interactions of grazing, nutrients and light in influencing autotroph species diversity have not previously been considered. Earlier studies have shown that grazing and nutrients interact in determining autotroph species diversity, since grazing decreases species diversity when nutrients (i.e. N or P) limit autotroph growth, but increases it when nutrients are replete. We hypothesized that increased light intensities would intensify the interactions between grazing and nutrients on algal species diversity, resulting in even stronger reductions in algal species diversity through grazing under nutrient–poor conditions, and to even stronger increases of algal species diversity through grazing under nutrient-rich conditions. We studied the effects of grazing (absent, present), nutrients (ambient, N + P enriched) and light (low light, high light) on benthic algal diversity and periphyton C:nutrient ratios (which can indicate algal nutrient limitation) in a factorial laboratory experiment, using the gastropod grazer Viviparus viviparus. Grazing decreased algal biomass and algal diversity, but increased C:P and N:P ratios of periphyton. Grazing also affected periphyton species composition, by decreasing the proportion of Spirogyra sp. and increasing the proportion of species in the Chaetophorales. Grazing effects on diversity as well as on periphyton N:P ratios were weakened when nutrients were added (interaction between grazing and nutrients). Chlorophyll a (Chl a) per area increased with nutrient addition and decreased with high light intensities. Light did not increase the strength of the interaction between grazing and nutrients on periphytic algal diversity. This study shows that nutrient addition substantially reduced the negative effects of grazing on periphytic algal diversity, whereas light did not interact with grazing or nutrient enrichment in determining periphytic algal diversity.     

Environmental context determines multi‐trophic effects of consumer species loss

Loss of biodiversity and nutrient enrichment are two of the main human impacts on ecosystems globally, yet we understand very little about the interactive effects of multiple stressors on natural communities and how this relates to biodiversity and ecosystem functioning. Advancing our understanding requires the following: (1) incorporation of processes occurring within and among trophic levels in natural ecosystems and (2) tests of context‐dependency of species loss effects. We examined the effects of loss of a key predator and two groups of its prey on algal assemblages at both ambient and enriched nutrient conditions in a marine benthic system and tested for interactions between the loss of functional diversity and nutrient enrichment on ecosystem functioning. We found that enrichment interacted with food web structure to alter the effects of species loss in natural communities. At ambient conditions, the loss of primary consumers led to an increase in biomass of algae, whereas predator loss caused a reduction in algal biomass (i.e. a trophic cascade). However, contrary to expectations, we found that nutrient enrichment negated the cascading effect of predators on algae. Moreover, algal assemblage structure varied in distinct ways in response to mussel loss, grazer loss, predator loss and with nutrient enrichment, with compensatory shifts in algal abundance driven by variation in responses of different algal species to different environmental conditions and the presence of different consumers. We identified and characterized several context‐dependent mechanisms driving direct and indirect effects of consumers. Our findings highlight the need to consider environmental context when examining potential species redundancies in particular with regard to changing environmental conditions. Furthermore, non‐trophic interactions based on empirical evidence must be incorporated into food web‐based ecological models to improve understanding of community responses to global change.     

Keystone predator effects and grazer control of planktonic primary production

If prey species exhibit trade-offs in their ability to utilize resources versus their ability to avoid predation, predators can facilitate prey turnover along gradients of productivity, shifting dominance from edible to inedible prey (the keystone predator effect). I tested this model under controlled, laboratory conditions, using a model aquatic system composed of zooplankton as the top consumer, a diverse community of algae as prey, and nutrients as basal resources. Nutrient manipulations (low and high) were crossed with presence–absence of zooplankton. Results supported theoretical predictions. Algal biomass increased in response to enrichment regardless of predator presence/absence. However, predators and nutrients had an interactive effect on algal biomass and size structure. At the low nutrient level, algal-prey were dominated by edible forms and attained similar biomass regardless of zooplankton presence/absence. At the high level of enrichment, presence of zooplankton favored higher levels of algal biomass and shifted dominance to large, inedible taxa. At the termination of the experiment, I performed a series of lab-based assays on the resultant algal community in order to quantify trade-offs among algal size classes in maximal population growth rates (as a measure of competitive ability for nutrients) and susceptibility to zooplankton grazing. Assays provided support for a size-based keystone trade-off. Small size classes of algae displayed higher maximal growth rates but were more susceptible to grazing effects. Large size classes were protected from grazing but showed low rates of population growth in response to enrichment.     

Species traits and environmental conditions govern the relationship between biodiversity effects across trophic levels

Changing environments can have divergent effects on biodiversity–ecosystem function relationships at alternating trophic levels. Freshwater mussels fertilize stream foodwebs through nutrient excretion, and mussel species-specific excretion rates depend on environmental conditions. We asked how differences in mussel diversity in varying environments influence the dynamics between primary producers and consumers. We conducted field experiments manipulating mussel richness under summer (low flow, high temperature) and fall (moderate flow and temperature) conditions, measured nutrient limitation, algal biomass and grazing chironomid abundance, and analyzed the data with non-transgressive overyielding and tripartite biodiversity partitioning analyses. Algal biomass and chironomid abundance were best explained by trait-independent complementarity among mussel species, but the relationship between biodiversity effects across trophic levels (algae and grazers) depended on seasonal differences in mussel species’ trait expression (nutrient excretion and activity level). Both species identity and overall diversity effects were related to the magnitude of nutrient limitation. Our results demonstrate that biodiversity of a resource-provisioning (nutrients and habitat) group of species influences foodweb dynamics and that understanding species traits and environmental context are important for interpreting biodiversity experiments.     

The effects of grazing by gastropods and physical factors on the upper limits of distribution of intertidal macroalgae

Summary The cover of foliose algae is sparse to non-existent above a low-level algal zone on many shores in N.S.W., except in rock-pools. Above this algal zone, encrusting algae, mostly Hildenbrandia prototypus, occupy most of the primary substratum on sheltered shores. Experimental manipulations at midtidal levels were used to test hypotheses about the effects of grazing by molluses and of physical factors during low tide on this pattern of algal community structure.Fences and cages were used to exclude grazers: molluscs grazed under roofs and in open areas. Cages and roofs provided shade, and decreased the harshness of the environment during low tide: fences and open areas had the normal environmental regime.In the absence of grazers, rapid colonization of Ulva and slower colonization by other foliose algae occurred in all experimental areas. The rate of colonization by Ulva sporelings was initially retarded on existing encrusting algae, but after a few months, cover of Ulva equalled that on cleared rock.Most species of algae only grew to maturity inside cages, and remained as a turf of sporelings inside fences. No foliose algae grew to a visible size in open, grazed areas. Grazing thus prevents the establishment of foliose algae above their normal upper limit on the shore, but the effects of physical factors during low tide prevent the growth of algae which become established when grazers are removed. Physical factors thus limit the abundance of foliose algae at mid-tidal levels.The recolonization of cleared areas by Hildenbrandia was not affected by the presence of a turf of sporelings, nor by the shade cast by roofs, but was retarded in cages where mature algae formed a canopy. Even under such a canopy, Hildenbrandia eventually covered as much primary substratum as in open, grazed areas. This encrusting alga is able to escape from the effects of grazing by having a tough thallus, and by its vegetative growth which allows individual plants to cover a lot of substratum, and by the tendency for new individuals to start growing from small cracks and pits in the rock, which are apparently inaccessible to the grazers.Mature foliose algae are removed from the substratum by waves, and many individual plants died during periods of hot weather. Sporelings in a turf were eliminated, after experimental fences were removed, by the combined effects of macroalgal grazers, which invaded the areas, and microalgal grarers which ate the turt from the edges inwards.The results obtained here are discussed with respect to other studies on limits to distribution of intertidal macroalgae, and the role of grazing in the diversity and structure of intertidal algal communities. Some problems of these experimental treatments are also discussed.     

The effects of top–down versus bottom–up control on benthic coral reef community structure

While climate change and associated increases in sea surface temperature and ocean acidification, are among the most important global stressors to coral reefs, overfishing and nutrient pollution are among the most significant local threats. Here we examined the independent and interactive effects of reduced grazing pressure and nutrient enrichment using settlement tiles on a coral-dominated reef via long-term manipulative experimentation. We found that unique assemblages developed in each treatment combination confirming that both nutrients and herbivores are important drivers of reef community structure. When herbivores were removed, fleshy algae dominated, while crustose coralline algae (CCA) and coral were more abundant when herbivores were present. The effects of fertilization varied depending on herbivore treatment; without herbivores fleshy algae increased in abundance and with herbivores, CCA increased. Coral recruits only persisted in treatments exposed to grazers. Herbivore removal resulted in rapid changes in community structure while there was a lag in response to fertilization. Lastly, re-exposure of communities to natural herbivore populations caused reversals in benthic community trajectories but the effects of fertilization remained for at least 2 months. These results suggest that increasing herbivore populations on degraded reefs may be an effective strategy for restoring ecosystem structure and function and in reversing coral–algal phase-shifts but that this strategy may be most effective in the absence of other confounding disturbances such as nutrient pollution.     

Evaluating the multiple mechanisms underlying herbivore-algal interactions in streams

Stream algal responses to herbivory were investigated under different environmental conditions. Snail densities and nutrient concentrations were manipulated in experimental enclosures to document the influence of nutrient availability on the magnitude of algal responses to herbivory. Periphyton mats in other enclosures were subjected to physical disruption by artificial means to evaluate the influence of disturbance on algal abundance. The impact of herbivory on algal abundance decreased substantially with increase in water column nutrient concentrations. This result was explained by findings that: (1) algal accumulation was constrained by nutrient availability under ambient water quality conditions. (2) accumulation of most algal populations was stimulated by nutrient enrichment only under grazed conditions. Thus, snail grazing simultaneously exerted a negative impact on algal abundance, by removal and consumption of a portion of the periphyton mat, and a stimulative effect, by increasing the availability of nutrients to remaining cells. Algal responses to artificial disturbance indicated that stimulative effects of herbivory were caused by the physical disruption of thr assemblage rather than by other processes (e. g., nutrient regeneration). However, consumptive losses far outweighed stimulative effects on algal abundance under ambient nutrient conditions. The magnitude of these two antagonistic effects was comparable under enriched conditions because grazing had only slight effects on algal abundance. Thus, the importance of different mechanisms of algal-herbivore interactions is strongly influenced by ambient environmental conditions, a finding that has important implications for predicting the outcome of herbivore-algal interactions in ecosystems with pronounced temporal and spatial variation in biotic and abiotic conditions.     

Independent and interactive effects of nutrients and grazers on benthic algal community structure

Algal responses to nutrients, grazing by Helicopsyche borealis, and concurrent grazing by Helicopsyche and Baetis tricaudatus were examined in recirculating stream chambers. Alagl communities, dominated by Achnanthes minutissima, Cocconeis placentula, and Synedra ulna, were primarily phosphorus-limited. Algal populations responded after only 6 days of nutrient enrichment. Initially, both the adnate diatom Cocconeis and erect diatom Synedra showed positive response to nutrient enrichment. Accumulation of algal biomass between day 3 and 6 in the P enriched treatment was resulted primarily from the growth of Synedra, an overstory rosette-like diatom colony. Such a shift in dominant growth from adnate to erect diatoms is a general phenomenon in periphyton succession in the absence of disturbance. Algal species showed differential responses to an increase of Helicopsyche densities. The accrual rate of Achnanthes continuously decreased with increasing grazer densities. The accrual rates of both Cocconeis and Synedra declined but reached plateaus between medium and high grazing densities. Baetis effectively and exclusively depressed Synedra and had no significant impact on Cocconeis. After concurrent grazing, algal communities were mainly dominated by Cocconeis (approximately 80% of total algal biovolume). The grazer' s mouth structures, grazing efficiencies, and mobility may account for the differential effects of concurrent grazing on algal communities. Significant interactive effects of P and grazing by Helicopsyche indicated that both nutrient addition and grazing may exert significant impact on algal communities. However, grazing may have a much stronger effect on algae than nutrients. Our results indicate that enhancement of algal biomass by P was dampened by grazing activities and that P had no effect on algal biomass in the presence of grazers.     

A test of the Menge-Sutherland model of community organization in a tropical rocky intertidal food web

Summary Menge and Sutherland (1976) predicted that in physically benign habitats: (1) community structure will be most strongly affected be predation, (2) the effect of predation will increase with a decrease in trophic position in the food web, (3) trophically intermediate species will be influenced by both predation and competition, and (4) competition will occur among prey species which successfully escape consumers. These predictions were tested in a tropical rocky intertidal community on the Pacific coast of Panama. The most abundant mobile species included fishes and crabs, which occupied the top trophic level, and predaceous gastropods and herbivorous molluscs, which occupied intermediate trophic levels. The most abundant sessile organisms were encrusting algae, foliose algae, barnacles, and bivalves. Diets were broad and overlapping, and 30.3% of the consumers were omnivorous. Each consumer group had strong effects on prey occurring at lower trophic levels: (1) Fishes and crabs reduced the abundance of predaceous snails, herbivorous molluscs, foliose algae, and sessile invertebrates. (2) Predaceous gastropods reduced the abundance of herbivorous molluscs and sessile invertebrates. (3) Herbivorous molluscs reduced the abundance of foliose algae and young stages of sessile invertebrates, and altered relative abundances of the encrusting algae. The encrusting algae, although normally the dominant space occupiers, proved to be inferior competitors for space with other sessile organisms when consumers were experimentally excluded. However, the crusts escaped consumers by virtue of superior anti-herbivore defenses and competed for space despite intense grazing. Observations do not support the hypothesis that the trophically intermediate species compete. Hence, with the exception of this last observation, the predictions of the Menge and Sutherland model were supported. Although further work is needed to evaluate other predictions of the model in this community, evidence from this study joins an increasing body of knowledge supporting the model. Contradictory evidence also exists, however, indicating that aspects of the model require revision.     

Light,nutrients and grazing interact to determine stream diatom community composition and functional group structure

1. Benthic algal communities are shaped by the availability of nutrients and light and by herbivore consumption. Many studies have examined how one of these factors affects algal communities, but studies simultaneously addressing all three are rare. 2. We investigated the effects of nutrients, light and a herbivore (the snail Potamopyrgus antipodarum) on benthic stream algae in a fully factorial experiment in 128 circular streamside channels. Four nutrient levels (none added to highly enriched), four snail grazing pressures (no snails to 777 individuals m^?2) and two light levels (ambient and 65% reduced) were applied. Colonising algae were dominated by diatoms (Bacillariophyta), which were determined to species using acid‐cleaned samples and assigned to functional groups according to their physiognomic growth form. 3. Diatom community structure changed considerably in response to our manipulations. Light had the strongest influence (as indicated by manova effect size), whereas nutrients had an intermediate effect and grazing was fairly weak. Relative abundances of six common diatom taxa decreased under reduced light, whereas five others became more prevalent. Eight taxa benefitted from nutrient enrichment, while three became rarer. Grazing affected the relative density of only one common taxon, which increased at higher grazing pressure. 4. Diatom functional groups also responded strongly. ‘Low profile’ taxa dominated at low resource levels (nutrients and especially light), whereas ‘high profile’ and ‘motile’ taxa became markedly more prevalent at higher resource levels. 5. Two‐way interactions between experimental factors were quite common. For example, Planothidium lanceolatum and Rossithidium petersenii responded more strongly to nutrient enrichment at reduced than at ambient light, whereas Cocconeis placentula responded more strongly at ambient light. For diatom functional groups, the benefit of nutrient enrichment for ‘motile’ diatoms was greater at ambient than at reduced light. 6. Our results imply that multifactor experiments are required to determine the main forces driving the composition of benthic algal communities. Further, our findings highlight the considerable potential of using functional algal groups as indicators of changing environmental conditions to complement the traditional taxonomic approach.     

Phytoplankton periodicity in a subtropical lake (Lake Kinneret,Israel)

Lake kinneret is a subtropical monomictic lake characterized by a Pyrrhophyta-Chlorophyta assemblage, supplemented by Cyanophyta in some years. Concerning their abundance and seasonal occurrence, the phytoplanktonic algae belong to two groups: algae appearing in quantity at a definite annual period and algae present throughout the year. Four stages of algal succession occur in the lake. There is a marked periodicity in the phytoplankton composition with a high standing stock in winter-spring, due to the dinoflagellate water-bloom, and a low one during the summer months, related to the high stability of summer stratification. The annual succession at the species level has been an almost constant event in the lake for many years.The increase in nutrient concentrations in 1973 and 1974 increased the diversity and abundance of algae (except Peridinium) but did not lead to significant changes in algal succession. Conversely, the decrease of the zooplankton grazing pressure in 1975 and 1976 facilitated the development of algal maxima during summer-fall. They were caused by nanoplanktonic forms, and they developed without additional enrichment of nutrients. The algal abundance and diversity decreased. The years 1981 and 1982 were characterized by both an increase in phosphorus and a decrease in zooplankton. These conditions favored the concomitant abundance of many species and an increase of non-Pyrrhophyta biomass.     

Effects of grazer immigration and nutrient enrichment on an open algae-grazer system

After disturbance, recovery dynamics of local populations depend on arrival rates of immigrants and local growth conditions. We studied the effects of herbivore immigration rates and nutrient enrichment on the dynamics of grazing insect larvae, benthic microalgae, and filamentous macroalgae recovering from low local densities in an open stream system. The two types of algae approximate a trade‐off between capabilities for growing at low resource levels and resisting herbivory. Many microalgae achieve relatively high growth rates at low nutrient levels but are vulnerable to grazers, whereas many macroalgae require high nutrient levels for growth but become increasingly defended with filament growth. We hypothesized that macroalgae should benefit more strongly than microalgae from increasing nutrient levels and decreasing grazer immigration rates, because both conditions increase macroalgal chances to grow into a size refuge from herbivory. We created a gradient of nutrient concentrations and manipulated drift immigration rates of macroinvertebrates. Macro‐ and microalgal biomass and the relative contribution of macroalgae to total algal biomass increased with increasing nutrient enrichment and decreased with increasing grazer immigration. Grazer densities responded positively to nutrient enrichment. The densities of large baetids responded positively to higher immigration rates of large baetids, whereas small baetids and chironomid larvae showed the opposite response. Per capita emigration of small baetids decreased with increasing algal biomass. The data suggest that large baetids negatively affected algal biomass and that small baetid and chironomid densities tracked resource levels set by nutrient enrichment and large baetids. Our experiments highlight the prospects of integrating disturbance with nutrient supply, immigration rates and local trophic interactions (determining recovery trajectories) into conceptual models of open system dynamics. We suggest that recovery trajectories towards micro‐ or macroalgal dominated states may depend on the spatial scale of disturbance relative to the movement ranges of migrating grazers and to nutrient supply.     

Grazer control of nutrient availability in the periphyton

Summary Benthic algal assemblages are regulated by both abiotic (e.g., nutrient) and biotic (e.g., grazing) constraint. The objective of this study was to determine how changes in these two factors affected the structure of an algal assemblage in an ephemeral stream. Coverslips were incubated for 21 days in enclosures containing one of three nutrient environments (ambient, phosphorus-enriched, or phosphorus and nitrogen enriched) and one of four densities of the snail Gonibasis (0, 40, 80, or 120 snails/m²) and examined directly to enumerate the algal assemblage. The effect of grazing on algal biomass was dependent on the nutrient environment. An overstory of diatoms was susceptible to removal by grazing and was not strongly affected by nutrient enrichment. An understory of Stigeoclonium was more resistant to grazing and responded strongly to nutrient enrichment only in the presence of grazers. Snail grazers may mediate nutrient availability to the understory indirectly by removing overlying cells or by direct excretion of nutrients. Multiple interactions occur between benthic herbivores and algae, and, as shown here, some of them are positive and involve modifications of the nutrient environment.     

Effects of grazing by Diadema antillarum Philippi (Echinodermata: Echinoidea) on algal diversity and community structure

In Discovery Bay, Jamaica W.I., densities of Diadema antillarum Philippi were experimentally manipulated over a range of 0–64 individuals · m^?2 with the aid of enclosures in shallow water. The effects of grazing by this echinoid on the algal community structure of a coral reef were compared under conditions of primary and post-primary succession.Algal biomass decreased as Diadema density was increased, particularly under primary succession conditions. A similar response was noted under post-primary conditions, but here biomass was also dependent on species composition and abundance of algae prior to experimental alterations of Diadema density. Peyssonnelia sp. was found to be particularly resistant to echinoid grazing, surviving even the most intense grazing. Within 11 months, algal diversity decreased significantly in an exponential fashion as Diadema density increased. This occurred under conditions of both primary and post-primary succession. Diversity, irrespective of index, was not maximized at intermediate grazing pressures. No single species of alga dominated the benthic community at even the lowest Diadema densities.Species composition of algae in the benthic community prior to changes in grazing pressure and lack of successful recruitment of a potential dominant were found to have a major influence on the response of the algal community to the treatments. Each of these factors can influence both the relationship between algal diversity and grazing pressure and the rate at which the community approaches an initial plateau of diversity in its earliest stages of succession.     

Light availability regulates the response of algae and heterotrophic bacteria to elevated nutrient levels and warming in a northern boreal peatland

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Timing of disturbance,top‐down,and bottom‐up driving on early algal succession patterns in a tropical intertidal community

Herbivory and nutrient enrichment are major drivers of the dynamics of algal communities. However, their effects on algal abundance are under the influence of seasons. This study investigated the effects of herbivory and nutrient enrichment on early algal succession patterns using cages (uncaged and fully caged treatments) and two nutrient levels (ambient and enriched concentrations). To determine seasonal influences, experiment plots on dead coral patches were cleared during both dry and rainy season. Of the 17 algal species recruited in the experiment plots, three were dominant: Ulva paradoxa C. Agardh, Padina in the Vaughaniella stage, and Polysiphonia sphaerocarpa Børgesen. In this succession process, U. paradoxa was the earliest colonizer and occupied the cleared plots within the first month after clearing with the highest percentage of 83.33 ± 1.67% to 88.33 ± 9.28%. Then, it was replaced by the late successional algae, Padina in the Vaughaniella stage, and P. sphaerocarpa. The effects of herbivory and nutrient enrichment on algal abundance varied across algal functional groups and seasons. During the dry season, neither herbivory nor nutrient enrichment affected Ulva cover but during the rainy season, Ulva cover was influenced by nutrient enrichment. However, the abundance of algae in this early stage was not apparently affected by either herbivory or nutrient enrichment. Our results indicated that the timing of disturbance strongly influenced the algal abundance and successional patterns in this tropical intertidal community.     

Nutrient loading and grazing by the minnow Phoxinus erythrogaster shift periphyton abundance and stoichiometry in mesocosms

1. Anthropogenic activities in prairie streams are increasing nutrient inputs and altering stream communities. Understanding the role of large consumers such as fish in regulating periphyton structure and nutritional content is necessary to predict how changing diversity will interact with nutrient enrichment to regulate stream nutrient processing and retention. 2. We characterised the importance of grazing fish on stream nutrient storage and cycling following a simulated flood under different nutrient regimes by crossing six nutrient concentrations with six densities of a grazing minnow (southern redbelly dace, Phoxinus erythrogaster) in large outdoor mesocosms. We measured the biomass and stoichiometry of overstory and understory periphyton layers, the stoichiometry of fish tissue and excretion, and compared fish diet composition with available algal assemblages in pools and riffles to evaluate whether fish were selectively foraging within or among habitats. 3. Model selection indicated nutrient loading and fish density were important to algal composition and periphyton carbon (C): nitrogen (N). Nutrient loading increased algal biomass, favoured diatom growth over green algae and decreased periphyton C : N. Increasing grazer density did not affect biomass and reduced the C : N of overstory, but not understory periphyton. Algal composition of dace diet was correlated with available algae, but there were proportionately more diatoms present in dace guts. We found no correlation between fish egestion/excretion nutrient ratios and nutrient loading or fish density despite varying N content of periphyton. 4. Large grazers and nutrient availability can have a spatially distinct influence at a microhabitat scale on the nutrient status of primary producers in streams.     

The effects of sea urchin grazing and drift algal blooms on a subtropical seagrass bed community

Subtropical seagrass beds can be subject to relatively high levels of direct herbivory and large blooms of drift algae, both of which can have important effects on the floral and faunal components of the community. Caging experiments were used to investigate these factors in a Thalassia testudinum bed in Biscayne Bay, Florida. Abundance of sea urchins, Lytechinus variegatus, and drift algae was manipulated within the cages. Naturally occurring levels of urchin grazing do not appear to affect the T. testudinum population. With experimentally increased urchin densities in the winter, seagrass shoot density and aboveground biomass decreased significantly. Similar effects were not detected in the summer, indicating that the impact of grazing on T. testudinum is lessened during this time of year. Shoot density was more vulnerable to grazing than aboveground biomass. This may be a result of grazing-induced increases in seagrass productivity, in which the remaining shoots produce more or longer leaves. In the winter, drift algal blooms form large mats that cover the seagrass canopy. Under the normal grazing regime these algal blooms do not have significant negative effects on the seagrass. With increased grazing pressure, however, there is a synergistic effect of grazing and drift algae on seagrass shoot density. At intermediate urchin density (10 per m(-2)), cages without algae did not undergo significant decreases in shoot density, while those with algae did. At the high density of urchins, the number of seagrass shoots in cages both with and without algae decreased, but the effect was more pronounced for cages with algae. Invertebrate abundance at the field site was low relative to other seagrass beds. There were no discernible effects, either positive or negative, of urchin and algae manipulations on the sampled invertebrate community.