Effects of consumers and enrichment on abundance and diversity of benthic algae in a rocky intertidal community

Human alteration of nutrient cycling and the densities of important consumers have intensified the importance of understanding how nutrients and consumers influence the structure of ecological systems. We examined the effects of both grazing and nutrient enrichment on algal abundance and diversity in a high-intertidal limpet-macroalgal community on the South Island of New Zealand, a relatively nutrient-poor environment. We used a fully factorial design with three levels each of grazing (manipulations of limpet and snail densities) and nutrients (nutrient-diffusers attached to the rock). Top-down control by grazers appears to be the driving organizing mechanism for algal communities in this system, with strong negative effects of grazing on algal diversity and abundance across all levels of nutrient enrichment. However, in contrast to the conclusions drawn from the analysis of the whole algal community, there was an interactive effect of grazing and enrichment on foliose algae, an important component of the algal system. When herbivory was reduced to very low levels, enrichment generated increases in the abundance and biomass of foliose algae. As expected, top-down control was the primary determinant of algal community structure in this system, controlling abundance and diversity of macrophytes on the upper shore. Contrary to expectations, however, increased nutrients had no community-wide effects, although foliose algal abundance increases were greatest with high nutrients and reduced grazing. It seems likely that most of the corticated algal species have limited capacity to respond to nutrient pulses in this nutrient-poor environment.     

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.     

Biodiversity effects on productivity and stability of marine macroalgal communities: the role of environmental context

The influence of biodiversity on ecosystem functioning has been the focus of much recent research, but the role of environmental context and the mechanisms by which it may influence diversity effects on production and stability remain poorly understood. We assembled marine macroalgal communities in two mesocosm experiments that varied nutrient supply, and at four field sites that differed naturally in environmental conditions. Concordant with theory, nutrient addition promoted positive species richness effects on algal growth in the first mesocosm experiment; however, it tended to weaken the positive diversity relationship found under ambient conditions in a second experiment the next year. In the field experiments, species richness increased algal biomass production at two of four sites. Together, these experiments indicate that diversity effects on algal biomass production are strongly influenced by environmental conditions that vary over space and time. In decomposing the net biodiversity effect into its component mechanisms, seven of the eight experimental settings showed positive complementarity effects (suggesting facilitation or complementary resource use) countered by negative selection effects (i.e. enhanced growth in mixture of otherwise slow growing species) to varying degrees. Under no conditions, including nutrient enrichment, did we find evidence of positive selection effects commonly thought to drive positive diversity effects. Species richness enhanced stability of algal community biomass across a range of environmental settings in our field experiments. Hence, while species richness can increase production, enhanced stability is also an important functional outcome of maintaining diverse marine macroalgal communities.     

The significance of predation in nest site selection of turtles: an experimental consideration of macro- and microhabitat preferences

Conceptual models predict counteractive effects of herbivores and nutrient enrichment on plant diversity and reversed effects of grazers under different nutrient regimes. I tested these hypotheses in 11 field experiments with periphyton communities in three different aquatic habitats (a highly eutrophic lake, an meso-eutrophic lake, and an meso-eutrophic part of the Baltic Sea coast) and in different seasons. Grazer access and nutrient supply were manipulated in a factorial design. Species richness and evenness were chosen as response variables. Both manipulated factors had significant and contrasting effects on diversity, with variable effect strength between sites and seasons. From the two aspects of diversity, evenness well reflected the changes in community composition. Fertilization tended to increase the dominance of few species and thus to decrease evenness, whereas grazers counteracted these effects by removing dominant life forms. The response of species richness was not as expected, since grazers decreased richness throughout, whereas nutrients had weaker effects but tended to increase richness. Species richness rather reflected changes in periphyton architecture. Grazers reduced algal richness presumably by co-consumption of rare species in the tightly connected periphyton assemblages, whereas enrichment may increase richness by providing more structure via increased dominance of filamentous species. Although grazer and nutrient effects on richness and evenness were opposing, there was no change in the effect of one factor by manipulation of the other.     

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.     

Grazing effects in macroalgal communities depend on timing of patch colonization

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

Interactions among irradiance,nutrients, and herbivores constrain a stream algal community

Using stream-side, flow-through channels, I tested for the effects of nutrients (NU) (nitrogen plus phosphorus), irradiance (L), and snail grazing (G) on a benthic algal community in a small, forested stream. Grazed communities were-dominated by a chlorophyte (basal cells ofStigeoclonium) and a cyanophyte (Chamaesiphon investiens), whereas ungrazed communities were comprised almost entirely of diatoms, regardless of nutrient and light levels. Snails maintained low algal biomass in all grazed treatments, presumably by consuming increased algal production in treatments to which L and NU were increased. When nutrients were increased, cellular nutrient content increased under ambient conditions (shaded, grazed) and biomass and productivity increased when snails were removed and light was increased. Together, nutrients and light had positive effects and grazing had negative effects on biomass (chlorophylla, AFDM, algal biovolume) and chlorophyll-and areal-specific productivity in ANOVAs. However, in most cases, only means from treatments in which all three factors were manipulated (ungrazed, +NU&L treatments) were significantly different from controls; effects of single factors were generally undetectable. These results indicate that all three factors simultaneously limited algal biomass and productivity in this stream during the summer months. Additionally, the effects of these factors in combination were in some cases different from the effects of single factors. For example, light had slight negative effects on some biomass parameters when added at ambient snail densities and nutrient concentrations, but had strong positive effects in conjunction with nutrient addition and snail removal. This study demonstrates that algal biomass and productivity can be under multiple constraints by irradiance, nutrients, and herbivores and indicates the need to employ multifactor experiments to test for such interactive effects.     

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.     

Effects of temporal and spatial heterogeneities created by consumer-driven nutrient recycling on algal diversity

A spatially explicit plant-herbivore model composed of planktonic herbivores, algal preys and nutrients was constructed to examine the effects of consumer-driven nutrient recycling (CNR) on the algal species richness with and without spatial structure. The model assumed that either of two essential nutrients (N and P) limited growth of algal populations and that consumer individuals moved randomly in the lattice and grazed all the algal species with the same efficiency. The results showed that when there was no CNR, the number of persistent algal species was affected by neither supply rates of external nutrients nor spatial structure and was consistently low. When consumers recycled nutrients according to their stoichiometry, the algal species richness changed with supply rates of external nutrients depending on spatial structure: the algal species richness decreased with increasing nutrient loadings when there were no spatial structure because CNR increased the probability of stochastic extinction of algal species by amplifying the oscillation of algae-consumer dynamics. However, when spatial structures were created by the migration of consumers, CNR increased the algal species richness in a range of nutrient loadings because spatial variation of grazing pressure functioned to stabilize the algal-consumer dynamics. The present study suggests that through grazing and nutrient recycling, consumer individuals can create ephemeral heterogeneity in growth environments for algal species and that this ephemerality is one of the keys to understanding algal species in nature.     

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.     

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.     

A gradient of nutrient enrichment reveals nonlinear impacts of fertilization on Arctic plant diversity and ecosystem function

Rapid environmental change at high latitudes is predicted to greatly alter the diversity, structure, and function of plant communities, resulting in changes in the pools and fluxes of nutrients. In Arctic tundra, increased nitrogen (N) and phosphorus (P) availability accompanying warming is known to impact plant diversity and ecosystem function; however, to date, most studies examining Arctic nutrient enrichment focus on the impact of relatively large (>25x estimated naturally occurring N enrichment) doses of nutrients on plant community composition and net primary productivity. To understand the impacts of Arctic nutrient enrichment, we examined plant community composition and the capacity for ecosystem function (net ecosystem exchange, ecosystem respiration, and gross primary production) across a gradient of experimental N and P addition expected to more closely approximate warming‐induced fertilization. In addition, we compared our measured ecosystem CO₂ flux data to a widely used Arctic ecosystem exchange model to investigate the ability to predict the capacity for CO₂ exchange with nutrient addition. We observed declines in abundance‐weighted plant diversity at low levels of nutrient enrichment, but species richness and the capacity for ecosystem carbon uptake did not change until the highest level of fertilization. When we compared our measured data to the model, we found that the model explained roughly 30%–50% of the variance in the observed data, depending on the flux variable, and the relationship weakened at high levels of enrichment. Our results suggest that while a relatively small amount of nutrient enrichment impacts plant diversity, only relatively large levels of fertilization—over an order of magnitude or more than warming‐induced rates—significantly alter the capacity for tundra CO₂ exchange. Overall, our findings highlight the value of measuring and modeling the impacts of a nutrient enrichment gradient, as warming‐related nutrient availability may impact ecosystems differently than single‐level fertilization experiments.     

A continental scale evaluation of the role of limpet grazing on rocky shores

It is critical for our knowledge of biodiversity and ecosystem processes to understand how individual species contribute to ecosystem processes and how these contributions vary in space and time. We used a manipulative field experiment in five locations over 17° of latitude [from southern Portugal to the Isle of Man (British Isles)] to determine the relative response of rocky intertidal algal assemblages released from control by the grazing of limpets. Response ratios showed that when limpets were removed there was a trend of effects from north to south. In the north, grazing had a strong effect on algal assemblages, but removing grazers reduced spatial variability in assemblages. In the south, the effect of limpet grazing was far weaker and removal of grazers had a much reduced impact on spatial variability. Here we show a clear trophic control of an ecosystem in that grazing by limpets not only determines macroalgal abundance overall but also modifies ecosystem stability via variability in cover of algae.     

Grazing increases functional richness but not functional divergence in Tibetan alpine meadow plant communities

Plant community diversity and ecosystem function are conditioned by competition among co-occurring species for multiple resources. Previous studies suggest that removal of standing biomass by grazing decreases competition for light, but coincident grazing effects on competition for soil nutrients remain largely unknown in Tibetan rangelands where grazing tends to deplete soil phosphorus availability. We measured five functional traits indicative of plant productivity and stoichiometry leaf carbon concentration (LCC), leaf nitrogen concentration (LNC), leaf phosphorus concentration (LPC), specific leaf area (SLA), leaf dry matter content (LDMC) for component species of plant communities in grazed and ungrazed plots in five Tibetan alpine meadows. We examined the diversity of traits singly Rao index of functional diversity (FDrao) and in aggregate functional richness (FRic), functional divergence (FDiv), and functional evenness (FEve) in response to grazing. We tested whether foliar trait diversity increases with nutrient competition but decreases with light competition when competitive exclusion is reduced by grazing. The FDrao of LPC significantly increased under grazing, but FDrao for LCC, LNC and SLA tended to decrease. The FDrao of LDMC increased at the drier site but decreased at the wettest site. There was a strong negative association between increase in FDrao of LPC and decrease in soil nutrients, especially soil phosphorus availability. The FRic for all five traits together increased with species diversity following grazing, but neither FDiv nor FEve differed significantly between grazed and ungrazed plots at most sites. Grazing in Tibetan alpine meadows tends to increase competition for soil phosphorus while decreasing competition for light, resulting in an increase in the functional richness in grazed plant communities without any significant changes in the overall functional diversity of foliar traits. Our study highlights the potential importance of grazing mediated competition for multiple resources in alpine meadow ecosystems.     

Epiphyte grazing enhances productivity of remnant seagrass patches

Anthropogenic nutrient enrichment is increasingly modifying community structure and ecosystem functioning in terrestrial and aquatic ecosystems. In marine ecosystems, the paradigm is that nutrient enrichment leads to a decline of seagrasses by stimulating epiphytic algal growth, which shades and overgrows seagrasses. This ignores the potential for herbivores, which graze upon epiphytic algae, to partially or wholly counter such nutrient effects. We conducted a field experiment to assess the role that the trochid gastropod Calthalotia fragum plays in reducing nutrient impacts on the seagrass, Posidonia australis, in an urbanized Australian estuary, Botany Bay, Sydney. In a field experiment, where nutrient loading and grazer density were orthogonally manipulated, nutrient enrichment failed to promote epiphyte biomass or diminish growth and primary productivity of P. australis. To the contrary, nutrient enrichment enhanced photosynthesis of the seagrass in plots where the grazer was present at higher density. Epiphytic growth was negatively affected by increased C. fragum density, while P. australis shoot growth was positively influenced. Thus, in this study system, grazing appears to play a much greater role in determining seagrass primary productivity and above‐ground growth than moderate nutrient loading, suggesting that the interaction between grazers and nutrients depends on the relative levels of each. Our study contributes to a growing body of literature suggesting that effects of nutrient loading on benthic assemblages are not universally negative, but are dependent on the biotic and abiotic setting.     

Differential responses of plants,reptiles and birds to grazing management,fertilizer and tree clearing

The relative effects of tree clearing, increased livestock densities and nutrient enrichment have rarely been compared across markedly different organism types, but negative effects are generally predicted. In contrast, adoption of rotational grazing is thought to benefit biodiversity in pastures but there are few supporting data. We examined the response of native plants, birds and reptiles to livestock management in south‐eastern Australia. We selected 12 pairs of rotationally and continuously grazed farms. Two 1‐ha plots were established in native pastures on each farm, one cleared and the second still retaining woodland tree cover. Stocking rates, fertilizer histories and landscape tree cover varied among farms. The abundance and richness of all taxa was lower in cleared pastures. The less mobile organisms (reptiles and plants) were positively correlated with tree cover at landscape scales, but only when trees were present at the plot scale. This pattern was driven by a few observations in landscapes with approximately 50% tree canopy cover. Neither bird abundance nor richness was correlated with stocking rates or nutrient enrichment, but plant richness responded negatively to both. The response of reptiles varied, declining with nutrient enrichment but positively correlated with livestock densities. These responses may be partly interpreted within the context of prior filtering of species pools through long‐term grazing pressure. No taxa responded positively to rotational grazing management. We predict that reductions in livestock density and soil nutrients will directly benefit plants and less so reptiles, but not birds. Indirect benefits are predicted for birds and reptiles if management increases persistence of trees within paddocks. Although some forms of rotational grazing can increase woodland tree recruitment, rotational grazing in itself is unlikely to enhance diversity.     

Grazing dynamics in intertidal rockpools: Connectivity of microhabitats

Differences between rockpool and emergent rock communities are often attributed to their contrasting physical conditions. However, differences in grazing pressure between rockpools and open rock could also exert an important structuring role. Greater densities and/or the lack of tidal constraints on foraging may allow grazing intensity to be greater in rockpools. Here, wax discs were deployed to compare grazing intensity between rockpool and emergent rock habitats at each of three tidal heights on a moderately exposed shore in SW England. Grazing intensity was then examined in relation to herbivore density. Grazing intensity in pools was twice that on emergent rock, despite a lower density of herbivores in the rockpools. Of these herbivores, patellid limpets are the dominant grazers on rocky shores throughout the NE Atlantic and are recognised to have a major role in structuring intertidal communities. Thus, subsequent experiments focussed on the influence of limpets in determining the differences in consumer pressure between rockpools and emergent rock. Three alternative explanations were considered: (1) the effect of continuous immersion on grazing intensity in rockpools; (2) differences in limpet species abundance between the two habitats; (3) movement of limpets from emergent rock into pools to feed. The level of grazing pressure exerted by Patella ulyssiponensis (Gmelin), the predominant species living constantly immersed in rockpools, was similar to that of P. vulgata (Linnaeus) which is predominantly found on emergent rock. P. vulgata were observed moving from emergent rock into rockpools during high tide. Manipulative experiments confirmed that these foraging excursions resulted in a 2-fold increase in grazing intensity in the pools. Grazing activity of P. vulgata in rockpools was not consistent between sites and may be influenced by differences in wave exposure and/or the abundance of microbial resources. Elevated consumer pressure in rockpools may be an important factor influencing algal assemblages and probably explains the predominance of grazer resistant-species in these pools.     

Top-down and bottom-up control in an eelgrass–epiphyte system

Nutrient supply and the presence of grazers can control primary producers in aquatic ecosystems, but the relative importance of bottom-up and top-down effects remains inconclusive. We conducted a mesocosm experiment and a field study to investigate the independent and interactive effects of nutrient enrichment and grazing on primary producers in an eelgrass bed Zostera marina . Nutrient treatments consisted of ambient or enriched (2× and 4× ambient) concentrations of inorganic nitrogen and phosphate. Grazer treatments consisted of presence or absence of field densities of the common isopod Idotea baltica . We found strong and interacting effects of nutrients and grazing on epiphytes. Epiphyte biomass and productivity were enhanced by nutrient enrichment and decreased in the presence of grazers. The absolute amount of epiphyte biomass consumed by grazers increased under high nutrient supply, and thus, nutrient effects were stronger in the absence of grazing. The effects of grazers and fertilisation on epiphyte composition were antagonistic: chain-forming diatoms and filamentous algae profited from nutrient enrichment, but their proportions were reduced by grazing. Eelgrass growth was positively affected by grazing and by nutrient enrichment at moderate nutrient concentrations. High nutrient supply reduced eelgrass productivity compared to moderate nutrient conditions. The monthly measured field data showed a nitrogen limitation for epiphytes and eelgrass in summer, which may explain the positive effect of nutrient enrichment on both primary producers. Generally, the field data suggested the possibility of seasonally varying importance of bottom-up and top-down control on primary producers in this eelgrass system.     

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.     

Previous agonistic experience determines both foraging behavior and territoriality in the limpet Lottia gigantea (Sowerby)

Lottia gigantea, the owl limpet, is an algal gardener. Territorial individuals actively defend gardens, which consist of clearedareas in the intertidal zone upon which a thick algal filmdevelops and upon which the territory holder feeds. Smaller,nonterritory holders raid these gardens and graze the algalfilm. Territorial individuals must obtain an adequate ration without compromising the productivity of the garden. In contrast,a nonterritory holder grazing on another limpet's territorymust obtain an adequate ration before it contacts the territoryholder and is driven off. In the laboratory, replicate setsof 10 limpets were trained to behave territorially and nonterritorially.Training mimicked natural encounters between territorial andnonterritorial L. gigantea. Limpets given territorial trainingleft significantly (t = -4.92, df = 9, p =.00041) more algalcover behind when grazing (on average 71%) than did limpetstrained to be nonterritorial (on average 50%). Territorial limpets seldom grazed over the same area more than once (4% of the grazedarea). In contrast, nonterritorial limpets frequently foragedover an area more than once; of the area grazed, 20% had beenvisited more than once. Previous agonistic experience determinesboth territorial behavior and foraging strategies, two of thecritical behaviors necessary for successful gardening behavior.Nonterritorial limpets maximize consumption per unit area, whereas territorial limpets appear to forage prudently, leaving a significantly greater proportion of the plant biomass behind.