Biomass and oxygen dynamics of the epiphyte community in a Danish lowland stream

SUMMARY. 1. We examined the abundance and oxygen metabolism of epiphytic organisms on the dominant macrophyte, Potamogeton pectinatus , in headwaters of the eutrophic River Suså. Microbenthic algae were abundant in the stream during spring and macrophytes during summer.
2. The low macrophyte biomass in spring supported a dense epiphyte cover whereas the high macrophyte biomass during summer had a thin epiphyte cover of 10–100-fold lower abundance per unit area of macrophyte surface. The epiphyte community was dominated by microalgae in spring and by heterotrophs, probably bacteria, during summer. This seasonal shift was shown by pronounced reductions of the chlorophyll a content (from 2–3% to 0.1–0.7% of organic DW), the gross photosynthetic rate (from 20–85 to 3–15 mg O₂, g-¹ organic DW h⁻¹) and the ratio of gross photosynthesis to dark respiration in the epiphyte community (from 5–18 to 1). The reduced contributions of epiphytic microalgae correlated with reduced light availability during summer.
3. Both the density and the photosynthetic activity of epiphytic algae were low on a stream area basis relative to those of microbenthic algae and macrophytes. Rapid variations in water velocity and extensive light attenuation in water and macrophyte stands probably constrained the development of epiphytic algae. The epiphyte community was more important in overall stream respiration, contributing c. 10% to total summer respiration and c. 20% to summer respiration within the predominantly heterotrophic microbial communities on sediments and macrophyte surfaces.     

Response of seagrass epiphyte loading to field manipulations of fertilization, gastropod grazing and leaf turnover rates

This study evaluates the bottom-up and top-down controls on epiphyte loads under low nutrient additions. Nutrients and gastropod grazers were manipulated in a field experiment conducted within a Thalassia testudinum meadow in Florida Bay, FL, USA. The effect of seagrass leaf turnover rate on epiphyte loading was also evaluated using novel seagrass short-shoot mimics that “grow,” allowing for the manipulation of leaf turnover rates. During the summer growing season and over the course of one seagrass leaf turnover period, low-level water column nutrient additions increased total epiphyte load, epiphyte chlorophyll a, and epiphyte autotrophic index. T. testudinum leaf nutrients (N and P) and leaf productivity also increased. Epiphyte loading and T. testudinum shoot biomass and productivity did not respond to a 60% mean increase in gastropod abundance. Manipulations of seagrass leaf turnover rates at minimum wintertime and maximum summertime rates resulted in a 20% difference in epiphyte loading. Despite elevated grazer abundances and increased leaf turnover rates, epiphyte loads increased with nutrient addition. These results emphasize the sensitivity of T. testudinum and associated epiphytes to low-level nutrient addition in a nutrient-limited environment such as Florida Bay.     

Development of an epiphyte indicator of nutrient enrichment: A critical evaluation of observational and experimental studies

An extensive review of the literature describing epiphytes on submerged aquatic vegetation (SAV), especially seagrasses, was conducted in order to evaluate the evidence for response of epiphyte metrics to increased nutrients. Evidence from field observational studies, together with laboratory and field mesocosm experiments, was assembled from the literature and evaluated for a hypothesized positive response to nutrient addition. There was general consistency in the results to confirm that elevated nutrients tended to increase the load of epiphytes on the surface of SAV, in the absence of other limiting factors. In spite of multiple sources of uncontrolled variation, positive relationships of epiphyte load to nutrient concentration or load (either nitrogen or phosphorus) often were observed along strong anthropogenic or natural nutrient gradients in coastal regions. Such response patterns may only be evident for parts of the year. Results from both mesocosm and field experiments also generally support the increase of epiphytes with increased nutrients, although outcomes from field experiments tended to be more variable. Relatively few studies with nutrient addition in mesocosms have been done with tropical or subtropical species, and more such controlled experiments would be helpful. Experimental duration influenced results, with more positive responses of epiphytes to nutrients at shorter durations in mesocosm experiments versus more positive responses at longer durations in field experiments. In the field, response of epiphyte biomass to nutrient additions was independent of climate zone. Mesograzer activity was a critical covariate for epiphyte response under experimental nutrient elevation, but the epiphyte response was highly dependent on factors such as grazer identity and density, as well as nutrient and ambient light levels. The balance of evidence suggests that epiphytes on SAV will be a useful indicator of persistent nutrient enhancement in many situations. Careful selection of appropriate temporal and spatial constraints for data collection, and concurrent evaluation of confounding factors will help increase the signal to noise ratio for this indicator.     

附着生物对太湖沉水植物影响的初步研究

在水草生长比较旺盛的季节（5—6月）,以富营养化严重的太湖梅梁湾和水草较丰富的贡湖湾作为采样区域,研究了2种环境状态不同湖区附着生物的现存量及其对沉水植物的影响.结果表明:富营养化严重水域植物上附着生物的现存量较高,但不同种类植物间有所差异.附着生物显著抑制水生植物光合作用,6月的抑制作用高达91.9%以上.这种抑制作用的大小随附着生物量的增加而增强,且受宿主植物的影响.     

Quantifying the influence of small omnivorous fishes on seagrass epiphyte load

The influence of two cryptic, seagrass‐inhabiting omnivorous fishes, the fan‐bellied leatherjacket Monacanthus chinensis and the variable sabretoothed blenny Petroscirtes variabilis, on seagrass epiphyte biomass are described. Overall, M. chinensis significantly reduced epiphyte biomass by 35·1% after 18 h in experimental aquaria, whilst P. variabilis showed a non‐significant 15·7% reduction. It is concluded that some cryptic omnivorous species play an important role in epiphyte removal in seagrass beds.     

Leaf vs. epiphyte nitrogen uptake in a nutrient enriched Mediterranean seagrass (Posidonia oceanica) meadow

In situ nitrogen uptake by leaves and epiphytes was studied in a Mediterranean seagrass (Posidonia oceanica) meadow impacted from a fish farm and a pristine meadow, using ¹⁵NH₄ and ¹⁵NO₃ as tracers. In the impacted meadow both leaves and epiphytes yielded higher N concentrations and showed higher specific N uptake, suggesting a linkage between N uptake and its accumulation. Epiphytes took up N faster than leaves in relation to their corresponding biomass, but when assessed per unit area, N uptake was higher in leaves. Leaf N uptake was negatively correlated with epiphyte N uptake. With increasing epiphyte load on leaves, N leaf uptake decreased while N epiphyte uptake increased, indicating that epiphyte overgrowth hinders N uptake by P. oceanica leaves. Epiphyte contribution to total N uptake increased, while that of leaves decreased at the impacted meadow. However, 2-3 times less N was transferred daily from the water column to the benthic compartment, through seagrass and epiphyte uptake on total, at the impacted meadow. Therefore, it is probably still the loss of the key species - the seagrass - which plays the most important role in N cycling in seagrass ecosystems.     

Phlorotannins versus other factors affecting epiphyte abundance on the kelp Ecklonia radiata

We examined factors affecting the abundance and distribution of epiphytes (fouling) on the sublittoral kelp Ecklonia radiata. We first assessed the importance of phlorotannins as chemical defences against epiphytes by (a) correlating epiphyte loads on different parts of the thallus with the phlorotannin content of those tissues, and (b) experimentally testing the effects of variation in phlorotannin concentration against the settlement and growth of gametes of Ulva lactuca, a common epiphyte in the system. Tissue phlorotannin content was, at best, only weakly related to epiphyte loads, with r ² values typically <0.10. Inhibition of Ulva gametes only occurred at concentrations >10 mg l⁻¹, which is 5 orders of magnitude greater than levels of phlorotannins in the water column around beds of E. radiata, and 1–3 orders of magnitude greater than estimated levels in the boundary layer at the surface of the plant. We concluded that phlorotannins have a negligible impact on patterns of epiphytism on E. radiata, and proceeded to investigate other factors influencing the distribution and abundance of epiphytes. In our samples the relative age of different parts of the thallus was strongly correlated with epiphyte abundance, with epiphyte densities greatest on the oldest tissue and least on the youngest. Distal parts of the thalli also had greater epiphyte loads than basal parts. Field experiments in which kelp tissue was suspended at two heights in an E. radiata forest for varying lengths of time confirmed the importance of the length of time that the tissue was in the water, and its height in the water column, to the development of an epiphyte community. Comparison of epiphyte loads on tissue from primary (smooth) and secondary (rough) laminae in these experiments indicated that surface rugosity also affected fouling. Macroherbivores were rare on E. radiata, and abundances of mesofauna and epiphytes were positively related, suggesting that grazers were not important determinants of patterns of epiphyte abundance. Although phlorotannins have been previously suggested to play an important role as defences against epiphytes, we suggest that water-soluble compounds such as phlorotannins are less likely to be effective defences against epiphytes than non-polar metabolites, which can adhere to the surface of the producing organism.     

Similarity of epiphyte biomass distribution on Posidonia and artificial seagrass leaves

Epiphyte biomass on Posidonia australis Hook f. and artificial seagrass leaves was analysed in relation to distance from leaf tip at different time periods following the autumnal shedding. The distribution pattern indicated no significant difference in the exponential nature of biomass increase.Similarities between epiphyte biomass distribution on real and artificial leaves is consistent with the hypothesis that biomass distribution on real leaves is not influenced significantly by metabolic interactions between host plant and epiphytes. Although a time component is involved in epiphyte biomass accumulation, the major factor in determining epiphyte biomass distribution is the relative position along the leaf.     

Non‐consumptive effects of avian predators on fish behavior and cascading indirect interactions in seagrasses

Top–down impacts of avian predators are often overlooked in marine environments despite evidence from other systems that birds significantly impact animal distribution and behavior; instead, birds are typically recognized for the impacts of their nutrient rich guano. This is especially true in shallow seagrass meadows where restoration methods utilize bird perches or stakes to attract birds as a passive fertilizer delivery system that promotes the regrowth of damaged seagrasses. However, this method also increases the local density of avian piscivores that may have multiple unexplored non‐consumptive effects on fish behavior and indirect impacts to seagrass communities. We utilized laboratory and field experiments to investigate whether visual cues of avian predators impacted the behavior of the dominant demersal fish in seagrass habitats, the pinfish Lagodon rhomboides, and promoted cascading interactions on seagrass‐associated fauna and epiphytes. In laboratory mesocosms, pinfish displayed species specific responses to models of avian predators, with herons inducing the greatest avoidance behaviors. Avoidance patterns were confirmed in field seagrass meadows where heron models significantly reduced the number of fish caught in traps. In a long term field experiment, we investigated whether avian predators caused indirect non‐consumptive effects on seagrass communities by monitoring fish abundances, invertebrate epiphyte grazers, and the seagrass epiphytes in response to heron models, bird exclusions, and bird stakes. On average, more fish were recovered under bird exclusions and fewer fish under heron models. However, we found no evidence of cascading effects on invertebrate grazers or epiphytes. Bird stake treatments only displayed a simple nutrient effect where higher bird abundances resulted in higher epiphyte biomass. Our results indicate that although birds and their visual cues can affect fish and epiphyte abundance through non‐consumptive effects and nutrient enrichment, these impacts do not propagate beyond one trophic level, most likely because of dampening by omnivory and larger scale processes.     

Physical variables driving epiphytic algal biomass in a dense macrophyte bed of the St. Lawrence River (Quebec, Canada)

The variables affecting epiphyton biomass were examined in a sheltered, multispecies macrophyte bed in the St. Lawrence River. Alteration of light penetration, resulting from the presence of dense macrophytes forming a thick subsurface canopy, primarily determined epiphyton biomass. Seasonal decrease of water levels also coincided with major increases in biomass. Plant morphology was the next important variable influencing epiphytic biomass, whereas the contribution of other variables (sampling depth, macrophyte species, relative abundance of macrophytes, and temperature) was low. Groups of lowest epiphyte biomass (0.1–0.6 mg Chla g^–1 DW) were defined by the combination of a low percentage of incident light (<13% surface light) and simple macrophyte stem types found below the macrophyte canopy. Highest epiphyte biomass (0.7–1.8 mg Chla g^–1 DW) corresponded to samples collected in mid-July and August, under high irradiance (>20% surface light) and supported by ramified stems. Our results suggest that epiphyton sampling should be stratified according to the fraction of surface light intensity, macrophyte architecture, and seasonal water level variations, in decreasing order of influence.     

An assessment of the importance of emergent and floating-leaved macrophytes to trophic status in the Loosdrecht lakes (The Netherlands)

The potential importance of the six major emergent and floating-leaved macrophyte species in recycling of sediment phosphorus in the Loosdrecht lakes was studied. Representative plant samples were collected at the time of maximum biomass, and analysed for biomass and carbon, nitrogen and phosphorus contents. Species cover was determined by aerial photography.Total cover in the seven lakes studied ranged between 2 and 26 percent. For the four main species, biomass per unit area increased with lake trophic status. Consistent differences in C, N and P contents per unit biomass were not observed. Although cover values were small, significant amounts of C, N and P were contained in the macrophytes when compared with maximum sestonic content.Potential P loads from macrophyte decay were calculated. In Lake Loosdrecht, the P load represented 15 percent of current external P inputs. The potential importance of macrophyte decay to P recycling in the other lakes is greater.Decay of macrophyte species at the end of the growing season appears to affect autumnal nutrient and chlorophyll a levels in the water column of some lakes. The re-establishment of submerged species following lake restoration may increase the importance of this pathway in the lakes.     

Macrophytes and diatoms – major results and conclusions from the STAR project

The interactions between sensitivity and variability of macrophyte and diatom communities were evaluated as a research support of methodologies required by the Water Framework Directive. Slope and shading were identified as additional typological parameters improving links between unimpacted macrophyte communities and running water types. Two other studies demonstrated indication value of macrophytes for assessment of nutrient enrichment and hydromorphological degradation. The special exercises were realized within the STAR project to evaluate sources of variability/uncertainty in assessment methods based on macrophytes and diatoms. Sampling period and shading of the site were found as major factors affecting variability in macrophyte assessment results. Uncertainty of diatom assessment is predominantly associated with selection of site, substrate type and taxonomic identification. Further extension of indication systems and definition of macrophyte/diatom-specific typology of running waters are considered as the main aims of subsequent investigations.     

Contrasting functional traits maintain lichen epiphyte diversity in response to climate and autogenic succession

Aim Lichen epiphytes are important for biodiversity conservation and are also widely applied as environmental indicators. However, biogeographical and ecological knowledge underpinning lichen epiphyte conservation, and the use of lichens as indicators, is based primarily on a limited range of ‘macrolichen’ species. Wider trends in epiphyte biodiversity remain largely unexplored. This paper examines the community structure of lichen epiphytes on aspen (Populus tremula L.) in Scotland, including species across all functional groups and comprising, therefore, taxonomically difficult ‘microlichens’. Location Northern Britain (Scotland). Methods Epiphytes were sampled from 12 sites throughout Scotland and examined at two scales: between and within aspen stands. Species were classified into contrasting functional groups and ordination by detrended correspondence analysis was used to summarize community structure. Results Within aspen stands (between trees) epiphyte communities showed successional patterns related to tree age. These successional patterns changed predictably for stands aligned along a climatic gradient (between stands). Main conclusions A dual climatic–successional trend in epiphyte community structure is presented. Large‐scale trends in epiphyte diversity are explained as the local response of species with contrasting functional traits to climate and autogenic succession. Turnover of functional groups between stands is positively related to β‐diversity, and ecological limits to the frequency of contrasting functional groups are presented. Accordingly, the study and application of lichen species with similar functional traits may inadequately represent patterns of biodiversity. This prompts criticism of the currently accepted conservation strategy, i.e. (1) an emphasis in the conservation literature on ‘macrolichen’ species with similar ecologies and (2) the application of lichen indices over climatically variable geographical areas.     

New paradigms in tropical limnology: the importance of the microbial food web

Biological invasions of aquatic plants (i.e., macrophytes) are a worldwide phenomenon, and within the last 15 years researchers have started to focus on the influence of these species on aquatic communities and ecosystem dynamics. We reviewed current literature to identify how invasive macrophyte species impact fishes and macroinvertebrates, explore how these mechanisms deviate (or not) from the accepted model of plant–fish interactions, and assess how traits that enable macrophytes to invade are linked to effects on fish and macroinvertebrate communities. We found that in certain instances, invasive macrophytes increased habitat complexity, hypoxia, allelopathic chemicals, facilitation of other exotic species, and inferior food quality leading to a decrease in abundance of native fish and macroinvertebrate species. However, mechanisms underlying invasive macrophyte impacts on fish and macroinvertebrate communities (i.e., biomass production, photosynthesis, decomposition, and substrate stabilization) were not fundamentally different than those of native macrophytes. We identified three invasive traits largely responsible for negative effects on fish and macroinvertebrate communities: increased growth rate, allelopathic chemical production, and phenotypic plasticity allowing for greater adaptation to environmental conditions than native species. We suggest that information on invasive macrophytes (including invasive traits) along with environmental data could be used to create models to better predict impacts of macrophyte invasion. However, effects of invasive macrophytes on trophic dynamics are less well-known and more research is essential to define system level processes.     

Niche response surfaces and productivity estimates of macrophytes in lake nainital,Central Himalaya,India

The niche response surfaces of two macrophytes, Potamogeton pectinatus L. and Hydrilla verticillata (L.f.) Royle were characterized and an assessment was made of various methods used to estimate the annual net production of the macrophyte stand in Lake Nainital, (located at 1937 m in Central Himalaya). The niche response surface were characterized by plotting the biomass values of the macrophytes in relation to two variant: site (depth) and time (months). Although the niche response surfaces of the two macrophytes showed a broad overlap, they differed from each other in detail. Furthermore, individuals of the same species populations showed varying responses to variations in site and time in terms of growth phenology and amounts of biomass and productivity. In view of the inter- and intraspecies differences in growth responses to the gradients of site and time, the method of calculation which followed the biomass changes of each site separately and considered all the positive increments by all species gave the most accurate net primary productivity estimate of the macrophyte stand.     

Biological indicators track differential responses of pelagic and littoral areas to nutrient load reductions in German lakes

External nutrient loading was reduced over the past decades as a measure for improving the water quality of eutrophic lakes in western Europe, and has since been accelerated by the adoption of the European Water Framework Directive (WFD) in 2000 (EC, 2000). A variety of eutrophication-related metrics have indicated that the response of biological communities to this decreased nutrient loading has been diverse. Phytoplankton, a major component of the pelagic community, often responded rapidly, whereas a significant delay was observed for submerged macrophytes colonizing littoral areas. In this study we tested whether assessment methods developed for phytoplankton and macrophytes in lakes during Germany's implementation of the WFD reflect this differential response. An assessment of 263 German lakes confirmed that a lower ecological state was recorded when based on the biological quality element (BQE) for macrophytes than the BQE for phytoplankton during the investigated period (2003–2012). On average, lakes had a moderate ecological status for both phytoplankton and macrophyte BQEs, but differences of up to three classes were observed in single cases. Long-term data were available for five lowland lakes subject to strong reductions in phosphorus loading. Their phytoplankton-based assessments indicated a constant improvement of the ecological status in parallel to decreasing water phosphorus concentrations. In contrast, macrophyte-based assessments indicated a 10–20 year delay in their ecological recovery following nutrient load reduction. This delay was confirmed by detailed data on the temporal development of macrophyte species diversity and maximum colonization depths of two lakes after nutrient load reduction. We conclude that the available WFD assessment methods for phytoplankton and macrophyte BQEs are suitable to track the differential response of pelagic and littoral areas to nutrient load reductions in German lakes.     

Integrating multiple landscape-scale drivers in the lichen epiphyte response: climatic setting, pollution regime and woodland spatial-temporal structure

Aim To quantify the role of multiple biodiversity drivers – pollution, woodland structure and climate – controlling lichen epiphyte composition and diversity. Location  Scotland, north‐west Europe. Methods Four compatible datasets were assembled: site‐scale species distribution data (response) and base‐line modelled data on climate, pollution loads and extent of old‐growth woodland (explanatory variables). First, partial‐canonical correspondence analysis was used: (1) to compare the importance of environmental variables to pure spatial effects and (2) to partition the importance of environmental variables in explaining species composition. Secondly, patterns of species richness were investigated using multiple least‐squares regression. Results Old‐growth woodland was the most important control of species richness. Pollution was the most important explanatory variable for species composition. The impact of pollution on composition (and to a lesser extent on richness) is explained: (1) By recovery of lichens with declining SO₂ pollution, although with epiphyte composition shifted by the recent effects of N‐pollution and (2) By the limited spatial extent of severe pollution, and generally low‐to‐moderate pollution loads across our study area, combined with the positive effect of old‐growth woodland extent in controlling species richness. The effect of climate and old‐growth woodland on species composition covaried, supporting an interaction between habitat quality and climatic setting, which may be important in understanding the epiphyte response to climate change. Conclusions Advances in conservation planning will likely require an integrated approach to understanding simultaneous effects of multiple drivers, providing opportunities for integrated management strategies. Our study provides a preliminary example of this approach by combining three key biodiversity drivers into a single framework for lichen epiphytes. Thus, reducing pollution loads may make old‐growth woodland that currently exists in a polluted landscape available for colonization, thereby extending the available habitat for epiphytes, and facilitating an effective species response to climate change.     

Environmental and spatial correlates of community composition,richness and status of boreal lake macrophytes

We assessed the relative roles of natural covariates, human disturbance (water quality and catchment land use) together with geography in driving variation in aquatic macrophyte community composition, richness and status among 101 lakes in southern and central Finland. In addition to all species together, we studied different growth forms (i.e. emergent and submerged macrophytes and aquatic bryophytes) separately. Partial redundancy analysis (taxonomic composition) and partial least-squares regression (species richness and status index) were employed to display the share of variability in macrophyte assemblages that was attributable to the environmental factors (both natural and human-affected) and the spatial filters generated through principal coordinates of neighbor matrices (PCNM).Macrophyte community composition, richness and status were explained by natural covariates, together with joint effects of human disturbance variables and space. The contributions of pure fractions of human disturbance and space were mostly modest, albeit variable among macrophyte groups and status indices. Alkalinity, historical distributions, colour, dynamic ratio and lake area were most important natural covariates for macrophytes. Of those variables influenced by human, macrophytes were mostly explained by conductivity, total phosphorus, turbidity and chlorophyll-a.Our results demonstrate, as expected, that macrophytes are dominantly affected by local environmental variables, whereas dispersal-related processes seem not to be important at regional extent. Response of macrophyte growth forms to environment and space, however, varied significantly. Community composition and richness of emergent macrophytes showed congruent response to natural covariates and human disturbance. Aquatic bryophytes, which are rarely studied along lake macrophytes, responded stronger than other growth forms to human disturbance. Contrary to our expectations, ecological indices were not affected by dispersal-related processes, but were mainly explained by natural covariates. This study is the first to investigate spatial patterns in aquatic macrophytes derived bioassessment. Geographical structuring of environmental variables and regional extent negatively affected indices, suggesting that ecological status assessment needs further development.     

Role of macrophytes in the variability of zooplankton community structure in the littoral zone of shallow lakes

Distribution, diurnal variability, aggregation of zooplankton in the littoral zone of lakes and effect of various macrophyte species on the structure of its community are considered. It is shown that the horizontal migrations of zooplankton, both direct and reverse ones, are caused mainly by the pressure of fish. The effect of predacious zooplankton is less important and is directed mainly at small-sized species. The intensity of horizontal migrations of zooplankton decreases with depth, while the effect of shore avoidance is observed for the large-sized zooplankton species and is caused not only by the pressure of fish but also by other factors, most likely abiotic. The problem of interaction between macrophytes and zooplankton cannot be reduced to the role of macrophytes as a refuge. Allelopathic properties of macrophytes, competitive relations between separate zooplankton species in macrophyte thickets, as well as the effect of predacious invertebrates associated with macrophytes on zooplankton remain unclear. The role of macrophytes as a factor causing changes in hydrodynamic processes in the littoral regions of lakes is also unknown.     

Relative effects of nutrient enrichment and grazing on epiphyte-macrophyte (Zostera marina L.) dynamics

The independent and interactive effects of nutrient concentration and epiphyte grazers on epiphyte biomass and macrophyte growth and production were examined in Zostera marina L. (eelgrass) microcosms. Experiments were conducted during early summer, late summer, fall, and spring in a greenhouse on the York River estuary of Chesapeake Bay. Nutrient treatments consisted of ambient or enriched (3× ambient) concentrations of inorganic nitrogen (ammonium nitrate) and phosphate. Grazer treatments consisted of the presence or absence of field densities of isopods, amphipods, and gastropods. epiphyte biomass increased with both grazer removal and nutrient enrichment during summer and spring experiments. The effect of grazers was stronger than that of nutrients. There was little epiphyte response to treatment during the fall, a result possibly of high ambient nutrient concentrations and low grazing pressure. Under low grazer densities of early summer, macrophyte production (g m^–2 d^–1) was reduced by grazer removal and nutrient enrichment independently. Under high grazer densities of late summer, macrophyte production was reduced by enrichment only with grazers absent. During spring and fall there were no macrophyte responses to treatment. The relative influence of epiphytes on macrophyte production may have been related to seasonally changing water temperature and macrophyte requirements for light and inorganic carbon.