![Potential indirect effects of predators on eelgrass (Zostera marina) production at Shelter Island, San Diego Bay, California, USA, given (A) the food web alteration hypothesis (negative effect), and (B) predator regulation of harmful epifauna (positive effect). Solid arrows denote negative direct effects, dashed arrows represent indirect effects, and font size represents relative abundance. Note that some epifauna may belong to multiple categories (foulers, algae grazers, and seagrass [SG] grazers). (C) Schematic representations of experimental treatments and (D) cage design with densities of microcarnivorous fishes (Paralabrax clathratus, Micrometrus minimus) used in predator treatments.](https://www.researchgate.net/profile/Todd-Anderson-12/publication/235713109/figure/fig1/AS:393570243629064@1470845866587/Potential-indirect-effects-of-predators-on-eelgrass-Zostera-marina-production-at_Q320.jpg)
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Top-down control of epifauna by fishes enhances seagrass production
Abstract and Figures
Predators can influence the structure and function of ecosystems by altering the composition or behavior of herbivore communities. Overexploitation of predators, therefore, may lead to habitat loss by altering important top-down interactions that facilitate habitat-forming species. In seagrass beds, top-down control of algal growth by mesograzers appears to facilitate seagrass production. The indirect consequences of higher-order trophic interactions, however, remain unclear. Although predators may limit the beneficial effects of algal mesograzers, it is also possible that they limit the abundance of invertebrates that consume and foul seagrasses. We used experimental enclosure and exclosure cages to explore the direct and indirect effects of microcarnivorous fishes on epifaunal invertebrates, epiphytic loads, and seagrass growth in a natural eelgrass (Zostera marina) bed in San Diego Bay, California, USA. Contrary to expectations, when fishes were excluded, invertebrate abundance increased by 300-1000%, fouling on eelgrass leaves increased by 600%, and eelgrass production declined by 50%. Despite high densities of predators in enclosures, subsequent effects did not differ from ambient conditions. When predators were excluded, however, abundances of epifauna (including tube-building crustaceans and an eelgrass-grazing limpet) increased dramatically, resulting in reduced seagrass production. Our results are supported by several studies of eelgrass communities in the northeastern Pacific, characterized by coastal upwelling, inverse estuaries, and a voracious seagrass-consuming limpet. These strong, positive, indirect effects of microcarnivores on seagrass production contrast with the beneficial mesograzer paradigm, highlighting the need for hypotheses to be tested across a variety of ecosystems with varying biophysical characteristics.
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... Beyond the ecological relationships described in sea otter-seagrass trophic cascade, we also examined our data for other biotic and abiotic relationships known to influence seagrass communities in other regions. These included the role of mesopredatory fishes which can exert top-down pressure on epifauna (Jephson et al. 2008, Moksnes et al. 2008, Baden et al. 2010, Lewis and Anderson 2012, benthic macroalgae and diatoms (Heck et al. 2000, Valentine andDuffy 2006), sediment grain size (De Boer 2007) and light availability (Lee et al. 2007). (Harper andMorris 2004, NOAA 2019). ...
... Our hypotheses were based on trophic cascade theory, documented trophic cascades in the northeast Atlantic and Elkhorn Slough, CA, and sea otter foraging ecology in southeast Alaska and focused on direct relationships between variables. Specifically we hypothesized that sea otters would have a negative relationship with crab biomass (Hoyt 2015), crab and fish biomass would have a negative relationship with epifauna load (Moksnes et al. 2008, Baden et al. 2010, Lewis and Anderson 2012, Hughes et al. 2013, epifauna load would have a negative relationship with macroalgae cover, diatom cover and epiphyte load, and epiphyte load would have a negative relationship with seagrass biomass (Hughes et al. 2004, Heck and Valentine 2007, Jephson et al. 2008, Duffy et al. 2015. We also hypothesized that the bottom-up forces of light and water nitrate concentration would have a positive relationship with both seagrass biomass and epiphyte load . ...
... Together, the potential that epifauna are not a reliable food source for crabs and fishes and the likelihood that these species life history operate on different spatial scales may indicate that our assumption that crabs and fishes consume seagrass epifauna was flawed. Field and laboratory studies have tightly linked these mesopredators and seagrass epifauna (Jephson et al. 2008, Moksnes et al. 2008, Baden et al. 2010, Lewis and Anderson 2012, Hughes et al. 2013, however our results suggest that this may not be the case in southeast Alaska; yet another example of local abiotic and biotic conditions potentially influencing the importance of top-down forces (Duffy et al. 2015). It is also possible that our metric of epifauna biomass is not a good indication of grazing pressure. ...
The presence and strength of trophic cascades can be a function of the local abiotic environment and relative abundance of key species. The reintroduction and expansion of sea otters Enhydra lutris, a known keystone species in kelp ecosystems, in southeast Alaska provides a rare natural experiment to test the generality of a apex-predator-seagrass trophic cascades across a broad spatial scale. We conducted an in-depth sea-grass community survey at 21 sites spanning ~100 km with variable sea otter presence to test for patterns of alternating abundance and direct relationships between species indicative of trophic cascades. Our analysis revealed some of the trophic relationships predicted by the apex predator-seagrass trophic cascades theory, including a strong negative relationship between sea otters and crabs and many of the expected relationships between nitrate, seagrass, epiphytes and epifauna. Other expected relationships within a trophic cascade, however, were not supported-including no relationship between crabs and epifauna, a critical link in the trophic cascade. Given the lack of evidence for all hypothesized direct relationships, we conclude that a sea otter mediated trophic cascade may not be present in southeast Alaska and could be due to local scale factors including the spatial heterogeneity, low resource availability and non-linear food chains in southeast Alaska seagrass communities. However, correlation analyses suggest further interactions among biological and environmental variables in southeast Alaska seagrass communities, including a positive correlation between sea otters and seagrass biomass. These results suggest that the effects of recovering apex-predator populations may not be generalizable across regions and spatial scales, highlighting a need for local assessment on the ecology and management of these populations.
... In particular, the diversity and community structure of mobile and sessile epifauna within Neptune grass (Posidonia spp., dominant in the Mediterranean Sea), eelgrass (Zostera spp., globally widespread) and turtlegrass (Thalassia spp., chiefly distributed in Indo-Pacific and West Atlantic) meadows are relatively well documented (Virnstein et al. 1984, Knowles & Bell 1998, Sánchez-Jerez et al. 1999, Wong & Dowd 2015, Demers et al. 2016, McDonald et al. 2016, Tano et al. 2016, Boyé et al. 2017. Numerous published studies focus on plant-animal interactions and energy flows within seagrass meadows (Jernakoff & Nielsen 1998, Lepoint et al. 1999, Lewis & Anderson 2012, Hammerschlag-Peyer et al. 2013. Overall, seagrass systems are much less studied in the tropics (although see Ansari et al. 1991, Klumpp et al. 1992, Prieto et al. 2003, Unsworth et al. 2007, Leopardas & Nakaoka 2014, Tano et al. 2016, Cavalcante et al. 2019) than temperate latitudes (Hootsmans & Vermaat 1985, Edgar & Shaw 1995, Heck et al. 1995, Nakamura & Sano 2005, Polte et al. 2005a,b, Spivak et al. 2009, Gullström et al. 2012, Wong & Dowd 2015, Lefcheck & Duffy 2015, Lefcheck et al. 2016, Boyé et al. 2017, Wong 2018 or subtropical zones (Edgar 1990c, Connolly 1995, Lemmens et al. 1996, Jernakoff & Nielsen 1998, Alfaro 2006, Micheli et al. 2008 Number of studies conducted on epifaunal communities within particular marine habitats. ...
... Values are based on a search conducted in ISI Web of Science database in July 2020 using the terms "epifauna" and "epifaunal". Lewis & Anderson 2012, Hammerschlag-Peyer et al. 2013, McDonald et al. 2016, Douglass et al. 2018, Ledbetter & Hovel 2020. Limited tropical evidence does, however, suggest that latitudinal influences are likely less significant than differences between epifaunal communities at the level of microhabitat structure (Fraser et al. 2020a). ...
... Herbivores strongly inuence the biomass, productivity and community composition of seagrass meadows through both direct and indirect mechanisms (Duffy et al., 2003;Heck and Valentine, 2006;Lewis and Anderson, 2012;Atwood et al., 2015;Cernohorsky et al., 2015) Previous studies revealed that the Lakshadweep Archipelagos is a very productive ecosystem with diverse and abundant macroalgal assemblage with large patches of seaweeds and extensive dense seagrass beds (Jagtap et al., 1999). However, earlier studies have not worked out a zone-wise variation in the macroalgal assemblage for these islands. ...
... Dwarf perch rely on eelgrass beds during many life stages (Schultz 1993) and over the study period eelgrass has increased by approximately 150%. Of note, dwarf perch help control eelgrass-damaging epiphytes and grazers allowing for increased eelgrass growth and production (Lewis and Anderson 2012). With the observed increase in both eelgrass cover and dwarf perch density in San Diego Bay, we may be observing in situ large-scale positive feedback between a foundational, habitatforming species and a microcarnivore that lives in that habitat. ...
A time series of 22 samples collected in April and July were taken of the fish assemblages of San Diego Bay over 11 non-consecutive years from 1995 to 2019. Each sample consisted of four ecoregions using a variety of collection methods designed to assess all components of the ichthyofauna. These samples yielded a total of 525,288 fishes belonging to 90 species and weighing 3,507 kg over the 25-year period. Northern anchovy was the most abundant fish species comprising 41% of the total catch despite its virtual absence near the end of the survey period, followed by topsmelt, slough anchovy, shiner perch, and Pacific sardine. Round stingrays dominated in weight constituting more than 27% of the total biomass taken followed by spotted sand bass, and northern anchovy. Approximately 64% of all individual fish captured in San Diego Bay during this study were juveniles. In a canonical correlation analysis, temperature, distance from the mouth of the bay, and salinity accounted for nearly 93% of the variance in individual species abundances. In the time series analyses, we found all three potential temporal patterns of fish species abundance, biomass, and diversity, namely: 1) significant decreases over time, 2) significant increases over time, and 3) no significant change over time. Abundance of eight of the top 35 species (including northern anchovy, topsmelt, slough anchovy, and shiner perch) and all forage species combined decreased over the study. Two species increased significantly in abundance, spotted sand bass and dwarf perch. Whereas, total abundance, total biomass, species richness, Shannon diversity, and the majority (71%) of species abundances did not change over the 25-year period. Despite various environmental perturbations and the general trends of decreases in larval and fish abundance indices over the Southern California Bight in recent years, the stability in species richness and composition over time reflects the generally resilient nature of the fish assemblage structure of San Diego Bay that has been maintained by active management including restoration practices.
... Although the detrimental impacts that animals can have on restored habitats are being considered in some specific restoration efforts (e.g., in those that remove species that predate or graze on transplanted habitat formers), many animals perform a suite of vital functions that are necessary Forum for ecosystem persistence, enhance ecosystem resilience through assisting disturbance recovery, and drive many of the services that restoration actively seeks to enhance (figure 1; Halpern et al. 2007). Consideration of mutualisms with animals that enhance habitat-forming species growth and survival is particularly important for coastal habitats that are frequently disturbed by natural and anthropogenic sources that make restoration inherently difficult (Lewis and Anderson 2012, Renzi et al. 2019, Gagnon et al. 2020. One of the most well-known mutualisms likely to have significant benefits for coastal restoration initiatives exists between bivalves and coastal vegetation. ...
As efforts to restore coastal habitats accelerate, it is critical that investments are targeted to most effectively mitigate and reverse habitat loss and its impacts on biodiversity. One likely but largely overlooked impediment to effective restoration of habitat-forming organisms is failing to explicitly consider non-habitat-forming animals in restoration planning, implementation, and monitoring. These animals can greatly enhance or degrade ecosystem function, persistence, and resilience. Bivalves, for instance, can reduce sulfide stress in seagrass habitats and increase drought tolerance of saltmarsh vegetation, whereas megaherbivores can detrimentally overgraze seagrass or improve seagrass seed germination, depending on the context. Therefore, understanding when, why, and how to directly manipulate or support animals can enhance coastal restoration outcomes. In support of this expanded restoration approach, we provide a conceptual framework, incorporating lessons from structured decision-making, and describe potential actions that could lead to better restoration outcomes using case studies to illustrate practical approaches.
... org/ 10. 1007/ s00338-022-02298-9. links facilitated by invertivorous fishes therefore underpin fundamental processes of marine ecosystem functioning that can influence global recreational and commercial fishery stocks (Lewis and Anderson 2012;Fulton et al. 2020;Froese and Pauly 2021). ...
Invertivorous fishes are key middle-order consumers that connect energy flows across different trophic levels. However, the potential for distinct functional roles to exist within this trophic guild has not been satisfactorily explored to date, meaning that current assessments of ecosystem resilience are likely to over-estimate the level of functional redundancy within a given invertivorous fish assembly. Our study examined the foraging behaviour and microhabitat preferences of invertivorous fish communities within the productive canopy macroalgal meadows of Ningaloo Marine Park, Western Australia. Our aim was to identify foraging specialisations that could yield distinct functional roles for species belonging to the guild. We found that invertivorous fishes at this location were chiefly represented by species belonging to the Labridae, Lethrinidae and Mullidae families. Individual species demonstrated strong preferences for foraging within specific microhabitat types, suggesting that the guild can be grouped into three categories of foraging specialists: ‘canopy forager’, ‘generalist’ and ‘abiotic forager’. Our results highlight subtle niche partitioning of foraging microhabitats within the trophic guild of invertivorous fishes associated with tropical macroalgal meadows. Moreover, this partitioning is consistent across seasons, despite significant fluctuations in canopy structure and biomass. The resulting refinement of foraging specialisations allows us to identify the functional roles of invertivorous fishes and afford greater protection to individual species that might otherwise be considered functionally redundant. Our results will help to inform knowledge of the functional impact of particular species and their ecological specialisations and improve our understanding of trophic flows in marine food webs for appropriate management and conservation.
... Due to complex food webs made up of animals, plants and microbes, multilevel biotic interactions can exert important control on carbon cycling. For example, predators play a critical role in suppressing herbivore abundance, consequently facilitating the growth of primary producers through trophic cascades (Atwood et al., 2015;Finke & Denno, 2005;Lewis & Anderson, 2012;Silliman & Bertness, 2002). Furthermore, combined effects of grazing and fungal infection can substantially reduce salt marsh production (Daleo & Iribarne, 2009;Silliman & Newell, 2003). ...
Research into biotic interactions has been a core theme of ecology for over a century. However, despite the obvious role that biota play in the global carbon cycle, the effects of biotic interactions on carbon pools and fluxes are poorly understood. Here we develop a conceptual framework that illustrates the importance of biotic interactions in regulating carbon cycling based on a literature review and a quantitative synthesis by means of meta‐analysis. Our study focuses on blue carbon ecosystems—vegetated coastal ecosystems that function as the most effective long‐term CO2 sinks of the biosphere. We demonstrate that a multitude of mutualistic, competitive and consumer–resource interactions between plants, animals and microbiota exert strong effects on carbon cycling across various spatial scales ranging from the rhizosphere to the landscape scale. Climate change‐sensitive abiotic factors modulate the strength of biotic‐interaction effects on carbon fluxes, suggesting that the importance of biota‐mediated carbon cycling will change under future climatic conditions. Strong effects of biotic interactions on carbon cycling imply that biosphere‐climate feedbacks may not be sufficiently represented in current Earth system models. Inclusion of new functional groups in these models, and new approaches to simplify species interactions, may thus improve the predictions of biotic effects on the global climate.
... The deep areas of these bays often provide an extension to coastal habitats and can create seasonal variation in fish assemblages (Froeschke and Allen, 2006). Seagrass beds not only provide a variety of critical ecosystems services such as stabilizing sediment and promoting biodiversity (Fonseca and Cahalan, 1992), but they also provide food to epifaunal invertebrates and fishes, along with physical refuge (Lewis and Anderson, 2012). From our initial trapping study, gnathiids rarely inhabited embayments, which could be attributable to multiple factors. ...
The distribution and abundance of organisms is typically shaped by multiple biotic and abiotic processes. Micropredators are parasite-like organisms that are smaller than their hosts and/or prey and feed on multiple hosts during a given life stage. Unlike typical parasites, however, they spend much or most of their time free-living, associating only temporarily with hosts. In the ocean, micropredators can impact multiple fish species, and in particular can have significant lethal and sub-lethal effects on newly settled fish. Although gnathiid isopods are abundant and primary micropredators in coral reef ecosystems, their impacts are relatively unexplored within sub-tidal temperate rocky reefs. We investigated the distribution of juvenile gnathiid isopods along sub-tidal temperate rocky reefs and tested trap methodology. We also quantified both the sub-lethal and lethal impacts of feeding-stage juvenile gnathiid isopods on juvenile, post settlement reef fish, Heterostichus rostratus (giant kelpfish). We were most interested in determining the relationship between gnathiid infestation level and fish swimming performance, in particular swimming metrics relevant to predator avoidance maneuvers. We found that Gnathia tridens was present in rocky reefs rather than embayments along the Southern California coastline and that within rocky reefs, gnathiids occurred in the highest densities in lighted traps. Surprisingly, we observed almost no influence of fish size or gnathiid sub-lethal infestation level on ambient or burst swimming performance metrics. However, burst duration was reduced by gnathiid infestation, which is important in predator avoidance. There were significant differences in survivorship among small fish compared to large fish as a result of gnathiid infestation. Larger fish survived higher numbers of gnathiids than smaller fish, indicating that parasite-induced mortality is greater for smaller fish. Investigations of the effects of micropredators on subsequent predator-mediated mortality, including the susceptibility of fishes and their individual responses to micropredators, can further contribute to our understanding of processes affecting recruitment in resident reef fish populations. Further research, especially within temperate sub-tidal ecosystems, is needed to understand and highlight the overlooked importance of micropredation in shaping fish populations within a reefscape.
... That is, though the seagrass itself is considered the habitat and nursery for a great many species, and obviously food for apex species such as sea turtles and manatees, it is the epiphytes which provide a large part of the base of the food chain for micro-and meso-grazers. These grazers include but are not limited to amphipods, copepods, polychaete worms, molluscs, shrimp and herbivorous fishes [24][25][26][27][28][29][30]. These smaller grazers are then the next part of the food web including many larger fish species [2,6,7,20]. ...
This paper presents pigment-based chemotaxonomy as a rapid method for the analysis of seagrass
epiphyte communities and how that data may be applied to the assessment of the full seagrass ecosystem.
Pigments-based chemotaxonomy uses diagnostic pigments to determine the biomass, using chlorophyll-a
as a proxy, of microalgal taxa within phytoplankton or epiphyte communities. Seagrass samples were
taken from Florida Bay, USA and around the southern tip of Eleuthera Island in the Bahamas.
Data is presented which reveals.
A. The need for care during sampling in order to avoid losing epiphytes due to sloughing,
B. Consideration of the exact site of sampling with a given area,
C. Variations in epiphyte production and community makeup with respect to time of year,
D. Epiphyte loading variations along the length of a seagrass blade,
E. Potential effects of light (top-down) and grazing (bottom-up) on epiphyte communities,
F. The importance of diatoms on the seagrasses and macro-algae of Florida Bay,
G. The use of epiphytometers to monitor epiphyte production versus time, and
H. The strong variation in epiphyte communities around the southern tip of Eleuthera Island.
All of these results and discussion are presented in order to reveal the application of pigment-based
chemotaxonomy and epiphytometers (aka fake seagrass) in the assessment of seagrass epiphyte
communities
Keywords: Epiphytometers; Biodiversity; Atmospheric; Seagrass; Epiphyte; Cyanobacteria
In the Central Red Sea, relatively pristine coral reefs meet intense coastal development, but data on the effects of related stressors for reef functioning are lacking. This in situ study therefore investigated the independent and combined effects of simulated overfishing and eutrophication on settlement of reef associated invertebrates on light-exposed and -shaded tiles over 4 months. Findings revealed that at the end of the study period invertebrates had almost exclusively colonized shaded tiles, indicating that algae were superior settling competitors on light-exposed tiles. On the shaded tiles, simulated overfishing prevented settlement of hard corals, but significantly increased settlement of polychaetes, while simulated eutrophication only significantly decreased hard coral settlement relative to controls. The combined treatment significantly increased settlement of bryozoans and bivalves compared to controls and individual manipulations, but significantly decreased polychaetes compared to simulated overfishing. These results suggest settlement of polychaetes and hard corals as potential bioindicators for overfishing and eutrophication, respectively, and settlement of bivalves and bryozoans for a combination of both. Therefore, if investigated stressors are not controlled, phase shifts from dominance by hard corals to that by other invertebrates may occur at shaded reef locations in the Central Red Sea.
Amphipods are often dominant components of benthic marine communities and may exhibit taxon-specific differences in feeding behavior. As a result, variation in the composition of amphipod communities is an important metric for the interpretation of trophic dynamics in benthic marine ecosystems. Though previous studies of amphipod diets indicate functional diversity among taxa, few studies have examined whether these differences are detectible using time-integrated natural tracers of in situ feeding habits. We used stable isotope ratios of nitrogen (delta N-15) and carbon (delta C-13) to examine trophic structure among amphipod taxa belonging to 5 families in an eelgrass (Zostera marina) ecosystem in San Diego Bay, California. The relative contribution of sources of primary production to amphipod diets was further analyzed using a mixing model bracketed by 2 dominant sources of primary production in the system: eelgrass and algae. We detected significant differences in both delta C-13 and delta N-15 among amphipod taxa, indicating family-specific differences in feeding habits that generally agree with previous studies of amphipod diets. Hyalids fed almost exclusively on eelgrass, ischyrocerids and ampithoids tended to feed more on algae and eelgrass, respectively, and caprellids exhibited heterogeneous feeding on both algae and eelgrass. The relatively high delta N-15 value of oedicerotids suggested that this group was likely carnivorous. Our findings are in general agreement with previous descriptions of family-specific amphipod feeding behaviors, suggesting that stable isotopes are a useful tool for describing the functional roles of mesograzers in eelgrass ecosystems.
Based on the assumption that each trophic level acts as a single exploitative population, a model relating the trophic structure of ecosystems to their potential primary productivity is developed. According to the model, herbivory pressure should be most severe in relatively unproductive environments. With increased potential productivity, the role of predation in herbivore regulation should become more important and the impact of herbivory upon plant communities should decrease. In very productive environments, increase in herbivory pressure is again probable, at least in aquatic ecosystems. The predicted pattern of phytomass and predicted results of manipulations are compared with available data. A reasonable fit between predictions and observations is found, although the sparsity of data and methodological uncertainties weaken the corroboration in several cases. In terrestrial ecosystems, the present version of the model seems best applicable to the vertebrate branch of the grazing chain, whereas the a...
Nutrient loading can dramatically alter benthic communities and has been implicated in the worldwide decline of seagrass beds. Ongoing changes in food webs caused by overfishing could also contribute to seagrass decline. However, the interaction of these factors and the role of small invertebrate grazers in mediating them are poorly understood. We examined the relative impacts of nutrient loading and food web alteration on eelgrass Zostera marina L. community structure in Chesapeake Bay by manipulating nutrients, predatory crabs, and invertebrate grazers in field enclosures over 28 d in summer. Nutrient loading increased epiphyte accumulation early in the experiment, decreased eelgrass biomass, and reduced the abundance of the colonial tunicate Botryllus schlosseri. Grazers decreased epiphyte accumulation, altered the recruitment of sessile invertebrates, and sometimes damaged eelgrass via overgrazing. Crabs reduced the abundance of eelgrass, rind changed the species composition and abundance of grazers and sessile invertebrates. On average, the impacts of food web alterations and nutrient loading were comparable in magnitude and tended to be additive, rather than interactive. However, the distinct responses of different taxa in the community to the experimental treatments indicated that food web structure interacted with both bottom-up and top-down forces to determine overall community organization. These results highlight the importance of incorporating food web dynamics into seagrass conservation and management efforts.
We assessed the individual and combined effects of removing large predators and enriching water column nutrients on shoalgrass Halodule wrightii meadows in Big Lagoon, Florida, USA. To simulate the first-order effects of large predator reductions, we stocked 2.0 m(2) enclosures with elevated (similar to 3 to 4x ambient) densities of the omnivorous pinfish Lagodon rhomboides, the dominant fish in local seagrass habitats, and we supplemented N and P in the water column to nearly 3x ambient levels. Monthly determinations of water column nutrients and chlorophyll a (chl a), coupled with bimonthly measurements of leaf epiphyte biomass, seagrass growth and biomass, and beginning and ending comparisons of mesograzer abundance, were used to evaluate the effects of increasing nutrient supply and changing food web structure. Results showed significant predator and nutrient effects, although there were fewer consumer effects and more negative nutrient effects on seagrasses than in our previous experiments, which had shown that mesograzers ameliorated the harmful effects of elevated nutrients on seagrasses. Epiphyte proliferation in enrichment treatments did not occur; thus, algal overgrowth could not explain the negative effects of nutrient loading on seagrass biomass. Instead, nutrient loading resulted in nitrogen-rich shoalgrass, and it appears that this high-quality food stimulated pinfish herbivory. Elevated pinfish consumption of the enriched shoalgrass then resulted in the decline of seagrass biomass in enrichment enclosures. These results add additional complexity to understanding and predicting the effects of eutrophication in coastal waters.
Using small-scale field experiments, ecologists have demonstrated convincingly that species interactions often influence local community structure. Rigorously examining such large-scale processes, however, has created logistical and conceptual challenges. The developing discipline of macroecology has encouraged ecologists to address these challenges by recognizing the value of a broader spatial and temporal perspective. Among the most powerful tools to emerge has been the comparative-experimental approach. This approach uses identical experiments replicated along a broad spatial gradient to test the potential influence of changing abiotic and biotic conditions that are not amenable to manipulation.
The ability to utilize both water column and sediment nutrient sources in a complex habitat may provide eelgrass with a partial release from nutrient competition with epiphytes that have more efficient uptake kinetics and can reduce eelgrass growth, particularly in eutrophic habitats. The authors investigated the relative effects of dissolved inorganic nitrogen in the water column vs. the sediments, and herbivory by the common isopod Idotea resecata, on eelgrass growth and epiphyte biomass in an intertidal eelgrass bed in Padilla Bay, Washington. Eelgrass growth is affected both by sediment nitrogen resources and the higher order effects of epiphytes and their control by I. resecata. Growth of eelgrass leaves tended to increase in response to sediment fertilization; this trend was significant in April 1988 but not in August 1987. At both times, leaf growth rates demonstrated a saturation-type respoonse to sediment ammonium concentrations >100 μmol/L, providing further support for N-limitation of eelgrass growth over much of the range in ambient concentrations (30-137 μmol/L) in the sediment porewaters. Together, sediment ammonium concentrations and epiphyte biomass explained 71% of the variance in eelgrass leaf growth in August 1987. Consideration of sediment nitrogen, epiphytes, or herbivores alone is unlikely to yield a predictable understanding of the control of eelgrass primary productivity in nature. Results can be reconciled by considering herbivory by I. resecata. in the laboratory, the isopod reduced epiphyte biomass by 1/3 and in its absence, epiphyte biomass increased with increasing N concentrations in the water column and negatively affected eelgrass growth. -from Authors
The effect of epiphytes on eelgrass photosynthesis was measured at varying light intensities and HCO3− concentrations by means of the 14C-technique. Eelgrass was collected in Vellerup Vig, Denmark during October and November 1975.The epiphytes, mainly diatoms of the species Cocconeis scutellum Ehr., formed a crust several layers thick on the older leaves. The epiphytes reduced the photosynthetic rate of the leaves by acting both as a barrier to carbon uptake and by reducing light intensity. At optimal light intensity, the reduction was about 45% at 0.2 meq. HCO3−1−1 and it gradually decreased to nearly zero at 2.55 meq.1−1. At varying light intensity and a HCO3− concentration of 1.7 meq.1−1, corresponding to Vellerup Vig water, both effects of the epiphytes were seen. Above 7.2 mW cm−2, they caused a constant reduction of photosynthesis due to carbon deficiency. Below 7.2 mW cm−2, the reduction increased linearly to about 58% at 0.44 mW cm−2 corresponding to the increasing importance of shading from the epiphytes.Influence of epiphytic populations on photosynthesis and survival of aquatic macrophytes is discussed. It is suggested that macrophytes can limit the epiphytic stands by excreting algal antibiotics or by keeping a high replacement rate of photosynthetic tissues as illustrated by eelgrass in Vellerup Vig.