Baggett, L.P., K.L. Heck, Jr., T.A. Frankovich, A.R. Armitage, and J.W. Fourqurean. 2013. Stoichiometry, growth, and fecundity responses to nutrient enrichment by invertebrate grazers in sub-tropical turtlegrass (Thalassia testudinum) meadows. Marine Biology 160: 169-180.
Staszak, L.A. and A.R. Armitage. 2013. Evaluating salt marsh restoration success with an index of ecosystem integrity. Journal of Coastal Research 29: 410-418. Abstract
Madrid, E.N., A.R. Armitage, and A. Quigg. 2012. The response of photosystem II to soil salinity and nutrients in wetland plant species of the northwestern Gulf of Mexico. Journal of Coastal Research 28: 1197-1207. Abstract
Madrid, E.N., A. Quigg, and A.R. Armitage. 2012. Marsh construction techniques influence carbon capture by emergent and submerged vegetation in a brackish marsh in the northwestern Gulf of Mexico. Ecological Engineering 42: 54-63. Abstract
Valinoti, C.E., C.-K. Ho, and A.R. Armitage. 2011. Native and exotic submerged aquatic vegetation provide different nutritional and refuge values for macroinvertebrates. Journal of Experimental Marine Biology and Ecology 409: 42-47. Abstract
Armitage, A.R., T.A. Frankovich, and J.W. Fourqurean. 2011. Long term effects of adding nutrients to an oligotrophic coastal environment. Ecosystems 14: 430-444. Abstract
Baggett, L.P., K.L. Heck, Jr., T.A. Frankovich, A.R. Armitage, and J.W. Fourqurean. 2010. Nutrient enrichment, grazer identity and their effects on epiphytic algal assemblages: field experiments in sub-tropical turtlegrass (Thalassia testudinum) meadows. Marine Ecology Progress Series 406: 33-45. Abstract
Armitage, A.R. and J.W. Fourqurean. 2009. Stable isotopes reveal complex changes in trophic relationships following nutrient addition in a coastal marine ecosystem. Estuaries and Coasts 32: 1152-1164. Abstract
Armitage, A.R., V. Gonzalez, and P. Fong. 2009. Decoupling of nutrient and grazer impacts on a benthic estuarine diatom assemblage. Estuarine, Coastal and Shelf Science 84: 375-382. Abstract
Frankovich, T.A., A.R. Armitage, A.H. Wachincka, E.E. Gaiser, and J.W. Fourqurean. 2009. Nutrient effects on seagrass epiphyte community structure in Florida Bay. Journal of Phycology 45: 1010-1020. Abstract
Armitage, A.R., S.M. Jensen, J.E. Yoon, and R.F. Ambrose. 2007. Wintering shorebird assemblages and behavior in restored tidal wetlands in southern California. Restoration Ecology 15: 139-148. Abstract
Armitage, A.R., K.E. Boyer, R.R. Vance, and R.F. Ambrose. 2006. Restoring assemblages of salt marsh halophytes in the presence of a rapidly colonizing dominant species. Wetlands 26: 667-676. Abstract
Armitage, A.R. and P. Fong. 2006. Predation and physical disturbance by crabs reduce the relative impacts of nutrients in a tidal mudflat. Marine Ecology Progress Series 313: 205-213. Abstract
Armitage, A.R. and J.W. Fourqurean. 2006. The short-term influence of herbivory near patch reefs varies between seagrass species. Journal of Experimental Marine Biology and Ecology 339: 65-74. Abstract
Armitage, A.R., T.A. Frankovich, and J.W. Fourqurean. 2006. Variable responses within epiphytic and benthic microalgal communities to nutrient enrichment. Hydrobiologia 569: 423-435. Abstract
Gil, M., A.R. Armitage, and J.W. Fourqurean. 2006. Nutrients increase epifaunal abundance and shift species composition in subtropical seagrass beds. Hydrobiologia 569: 437-447. Abstract
Armitage, A.R., T.A. Frankovich, K.L. Heck, Jr., and J.W. Fourqurean. 2005. Experimental nutrient enrichment causes complex changes in seagrass, microalgae, and macroalgae community structure in Florida Bay. Estuaries 28: 422-434. Abstract
Armitage, A.R. and P. Fong. 2004. Gastropod colonization of a created coastal wetland: potential influences of habitat suitability and dispersal ability. Restoration Ecology 12: 391-400. Abstract
Armitage, A.R. and P. Fong. 2004. Upward cascading effects of nutrients: shifts in a benthic microalgal community and a negative herbivore response. Oecologia 139: 560-567. Abstract
Boyer, K.E., P. Fong, A.R. Armitage, and R.A. Cohen. 2004. Elevated nutrient content of tropical macroalgae increases rates of herbivory in coral, seagrass, and mangrove habitats. Coral Reefs 23:530-538. Abstract
In concept, ecological restoration will improve ecosystem characteristics of degraded habitats, but in practice, restoration success assessments typically target vegetation communities. We sought to determine if estuarine emergent marsh restoration projects in Galveston Bay, Texas that had successfully achieved permit-mandated plant coverage were comparable to reference sites at an ecosystem level. We used a Rapid Assessment Method developed specifically for this habitat (Galv-RAM) to compare restored (ages 5-15 years) and reference marshes. Thirteen biotic and abiotic characteristics were used to calculate an ecosystem index score, where pristine habitat would score 100%. The average Galv-RAM ecosystem index score for reference marshes (81%) was typical for urbanized estuaries. Restored marshes scored 75%, indicating that they were relatively well developed, although there was substantial variability in ecosystem index scores among sites. Discriminant function analysis revealed that reference sites had more benthic epifauna (periwinkles, fiddler crabs); epifauna were virtually absent from restored sites. Overall, Galv-RAM scores did not vary with restored marsh age, but some individual features changed over time: older restored sites tended to have higher plant diversity and belowground plant biomass than younger restored sites. The ultimate goal of coastal wetland restoration is to improve the integrity of the regional wetland landscape by augmenting many different ecosystem functions. Therefore, although not all measured variables scored optimally in all restored sites, each of the sites had relatively high ecological value and contributed to the integrity of a larger-scale matrix of wetland habitat.
Madrid, E.N., A.R. Armitage, and A. Quigg. 2012. The response of photosystem II to soil salinity and nutrients in wetland plant species of the northwestern Gulf of Mexico. Journal of Coastal Research 28: 1197-1207.
The photosynthetic response of many wetland plant species to soil salinity and nutrients has been described in the laboratory, but less is known about the cumulative effect of these abiotic factors in the natural environment. In this investigation we correlated field measurements of chlorophyll fluorescence with simultaneous measurements of soil nitrogen and phosphorus content, soil salinity, and relative leaf nitrogen content in four species (Spartina alterniflora Loisel, S. patens (Aiton) Muhl, Schoenoplectus californicus (C.A. Mey.) Steud. and S. robustus (Pursh) M.T. Strong), that are common in brackish and salt marshes of the northwestern Gulf of Mexico. We found that the range of environmental conditions experienced by each species in the field is greater than what has been considered in laboratory investigations, and that saline soils may elicit an inverse photochemical response that has not been documented in the laboratory. The relationship between chlorophyll fluorescence and soil N:P ratio was not significant in S. alterniflora, nonlinear in S. patens and S. robustus, and significant and positive in S. californicus. The relationship between leaf nitrogen content and effective quantum yield was significant and positive in S. alterniflora and S. robustus, but only S. alterniflora appeared to be able to increase relative leaf nitrogen content over a wide range of soil nutrient and salinity regimes. S. californicus had the greatest potential for photosynthetic light capture but also had the narrowest ecological distribution. Thus, the species best adapted to high levels of abiotic stress (S. alterniflora) was less dominant at lower salinities, and species with the highest potential for photosynthetic performance (S. robustus and S. californicus) were only found in locations with favorable abiotic conditions.
Madrid, E.N., A. Quigg, and A.R. Armitage. 2012. Marsh construction techniques influence carbon capture by emergent and submerged vegetation in a brackish marsh in the northwestern Gulf of Mexico. Ecological Engineering 42: 54-63.
Coastal marshes play an important role in global carbon cycles, yet coastal development has led to widespread losses of marsh habitat. To address this problem, many coastal wetlands have been restored or created over the past several decades using a variety of construction techniques, but it is unclear if net plant carbon capture in constructed marshes is equal to that of reference marshes, or if rates of plant carbon capture are influenced by marsh construction techniques. To comparatively assess relative carbon capture by emergent and submerged vegetation in constructed and reference marshes, we measured standing biomass and carbon content in above- and belowground emergent plant tissue and submerged vegetation in three constructed areas (2-3 years old) and one reference area in a brackish marsh in the northwestern Gulf of Mexico in 2009 and 2010. We also used aerial photographs to construct a GIS database of emergent and submerged vegetation coverage. These data were combined to estimate net annual plant carbon capture per square meter of marsh vegetation in each constructed and reference area. This index of carbon input to wetland vegetation suggests that rates of carbon capture by emergent aboveground vegetation and submerged aquatic vegetation were similar in constructed and reference areas. However, submerged vegetation captured less carbon (0.1-0.3 kg m(-2)) than emergent vegetation (0.2-1.7 kg m(-2)), and constructed areas contained an order of magnitude less emergent habitat than the reference area. Consequently, the annual carbon production of entire constructed areas (emergent + submerged vegetation; 0.1-1.2 kg m(-2)) was always less than half that of the reference area (0.8-2.5 kg m(-2)). Therefore, although productivity of emergent and submerged vegetation in constructed and reference areas was similar, the smaller ratio of land to water in the constructed areas reduced their annual rate of plant carbon capture at a larger spatial scale. To more closely mimic rates of plant carbon capture in reference marsh habitats, constructed marsh designs should aim to replicate the ratio of land to water in adjacent reference marshes.
Valinoti, C.E., C.-K. Ho, and A.R. Armitage. 2011. Native and exotic submerged aquatic vegetation provide different nutritional and refuge values for macroinvertebrates. Journal of Experimental Marine Biology and Ecology 409: 42-47.
Proliferation of exotic submerged aquatic vegetation (SAV) has the potential to alter trophic interactions among native fauna. The invasion of Myriophyllum spicatum (Eurasian watermilfoil) into brackish and freshwater wetlands on the Gulf Coast of the United States has decreased the abundance of native plant species such as Ruppia maritima (widgeongrass). Macroinvertebrates use SAV to meet nutritional requirements and seek refuge from predators. Shifts in SAV dominance can alter patterns of SAV use by associated fauna, but the dynamics of these alterations following Myriophyllum displacement of Ruppia are not well understood. In our study, we examined the trophic implications of shifts between native (Ruppia) and exotic (Myriophyllum) SAV in a brackish wetland in Port Arthur, Texas, USA. This study addressed the following questions: 1) Do aquatic invertebrates directly consume Myriophyllum, and 2) do predator–prey interactions differ in native and exotic SAV canopies? We collected Ruppia, Myriophyllum, Palaemonetes spp. (grass shrimp), and juvenile Callinectes sapidus (blue crabs) from the field and ran a series of three mesocosm experiments in the laboratory. In a “no-choice” grazing experiment, Palaemonetes consumed 13% of the Ruppia biomass but had no impact on Myriophyllum biomass. In a second study, when given a choice of plant canopies, Palaemonetes selected Ruppia over Myriophyllum 60% of the time when Callinectes was absent, but selected Myriophyllum more frequently in the presence of Callinectes. The selection of Myriophyllum was likely due to its refuge value; a third mesocosm experiment resulted in 60% Palaemonetes mortality due to Callinectes predation in Ruppia canopies but only 20% mortality in Myriophyllum canopies. Myriophyllum has a more structurally complex canopy that probably provides better protection from predation. Our studies suggest that native and exotic plants provide substantially different values as food sources and as refuges from predators for lower trophic levels.
Management of ecological disturbances requires an understanding of the time scale and dynamics of community responses to disturbance events. To characterize long-term seagrass bed responses to nutrient enrichment, we established six study sites in Florida Bay, USA. In 24 plots (0.25 m(2)) at each site, we regularly added nitrogen (N) and phosphorus (P) in a factorial design for 7 years. Five of the six sites exhibited strong P limitation. Over the first 2 years, P enrichment increased Thalassia testudinum cover in the three most P-limited sites. After 3 years, Halodule wrightii began to colonize many of the P-addition plots, but the degree of colonization was variable among sites, possibly due to differences in the supply of viable propagules. Thalassia increased its allocation to aboveground tissue in response to P enrichment; Halodule increased in total biomass but did not appear to change its aboveground: belowground tissue allocation. Nutrient enrichment did not cause macroalgal or epiphytic overgrowth of the seagrass. Nitrogen retention in the study plots was variable but relatively low, whereas phosphorus retention was very high, often exceeding 100% of the P added as fertilizer over the course of our experiments. Phosphorus retentions exceeding 100% may have been facilitated by increases in Thalassia aboveground biomass, which promoted the settlement of suspended particulate matter containing phosphorus. Our study demonstrated that low-intensity press disturbance events such as phosphorus enrichment can initiate a slow, ramped successional process that may alter community structure over many years.
Baggett, L.P., K.L. Heck, Jr., T.A. Frankovich, A.R. Armitage, and J.W. Fourqurean. 2010. Nutrient enrichment, grazer identity and their effects on epiphytic algal assemblages: field experiments in sub-tropical turtlegrass (Thalassia testudinum) meadows. Marine Ecology Progress Series 406: 33-45.
We tested the relative importance of top-down and bottom-up effects by experimentally evaluating the combined and separate effects of nutrient availability and grazer species composition on epiphyte communities and seagrass condition in Florida Bay. Although we succeeded in substantially enriching our experimental cylinders, as indicated by elevated nitrogen concentrations in epiphytes and seagrass leaves, we did not observe any major increases in epiphyte biomass or major loss of Thalassia testudinum by algal overgrowth. Additionally, we did not detect any strong grazer effects and found very few significant nutrient-grazer interactions. While this might suggest that there was no important differential response to nutrients by individual grazer species or by various combinations of grazers, our results were complicated by the lack of significant differences between control and grazer treatments, and as such, these results are best explained by the presence of unwanted amphipod grazers (mean = 471 ind. m(-2)) in the control cylinders. Our estimates of grazing rates and epiphyte productivities indicate that amphipods in the control cylinders could have lowered epiphyte biomass to the same level that the experimental grazers did, thus effectively transforming the control treatments into grazer treatments. If so, our experiments suggest that the effects of invertebrate grazing (and those of amphipods alone) were stronger than the effects of nutrient enrichment on epiphytic algae, and that it does not require a large density of grazers to control epiphyte biomass even when nutrient loading rates are substantially elevated.
Armitage, A.R. and J.W. Fourqurean. 2009. Stable isotopes reveal complex changes in trophic relationships following nutrient addition in a coastal marine ecosystem. Estuaries and Coasts 32: 1152-1164.
Complex links between the top-down and bottom-up forces that structure communities can be disrupted by anthropogenic alterations of natural habitats. We used relative abundance and stable isotopes to examine changes in epifaunal food webs in seagrass (Thalassia testudinum) beds following 6 months of experimental nutrient addition at two sites in Florida Bay (USA) with different ambient fertility. At a eutrophic site, nutrient addition did not strongly affect food web structure, but at a nutrient-poor site, enrichment increased the abundances of crustacean epiphyte grazers, and the diets of these grazers became more varied. Benthic grazers did not change in abundance but shifted their diet away from green macroalgae + associated epiphytes and towards an opportunistic seagrass (Halodule wrightii) that occurred only in nutrient addition treatments. Benthic predators did not change in abundance, but their diets were more varied in enriched plots. Food chain length was short and unaffected by site or nutrient treatment, but increased food web complexity in enriched plots was suggested by increasingly mixed diets. Strong bottom-up modifications of food web structure in the nutrient-limited site and the limited top-down influences of grazers on seagrass epiphyte biomass suggest that, in this system, the bottom-up role of nutrient enrichment can have substantial impacts on community structure, trophic relationships, and, ultimately, the productivity values of the ecosystem.
Strong interactions between top-down (consumptive) and bottom-up (resource supply) trophic factors occur in many aquatic communities, but these forces can act independently in some microphytobenthic communities. Within benthic estuarine diatom assemblages, the dynamics of these interactions and how they vary with abiotic environmental conditions are not well understood. We conducted a field experiment at two sites with varying habitat characteristics to investigate the interactive effects of grazers and nutrients on benthic estuarine diatoms. We crossed snail (Cerithidea californica) and nutrient (nitrogen and phosphorus)addition treatments in enclosures on a restored tidal sandflat and a reference tidal mudflat in Mugu Lagoon, southern California. We repeated the study in summer 2000 and spring 2001 to assess temporal variation in the interactions. Snails caused a large decrease in diatom relative abundance and biomass (estimated as surface area); nutrients increased diatom abundance but did not alter diatom biomass. Snails and nutrients both reduced average diatom length, although the nutrient effect was weaker and temporally variable, occurring in the reference mudflat in the spring. There were few interactions between snail and nutrient addition treatments, suggesting that links between top-down and bottom-up forces on the diatom community were weak. There were no consistent differences in diatom assemblage characteristics between the two study sites, despite marked differences in sediment grain size and other abiotic characteristics between the sites. The strong diatom response to herbivores and weaker responses to enrichment differed from the previous studies where cyanobacteria increased in response to nutrient enrichment, further dissolving the "black box" perception of microphytobenthic communities.
A field experiment was employed in Florida Bay investigating the response of seagrass epiphyte communities to nitrogen (N) and phosphorus (P) additions. While most of the variability in epiphyte community structure was related to uncontrolled temporal and spatial environmental heterogeneity, P additions increased the relative abundance of the red algae-cyanobacterial complex and green algae, with a concomitant decrease in diatoms. When N was added along with P, the observed changes to the diatoms and the red algae-cyanobacterial complex were in the same direction as P-only treatments, but the responses were decreased in magnitude. Within the diatom community, species relative abundances, species richness, and diversity responded weakly to nutrient addition. P additions produced changes in diatom community structure that were limited to summer and were stronger in eastern Florida Bay than in the western bay. These changes were consistent with well-established temporal and spatial patterns of P limitation. Despite the significant change in community structure resulting from P addition, diatom communities from the same site and time, regardless of nutrient treatment, remained more similar to one another than to the diatom communities subject to identical nutrient treatments from different sites and times. Overall, epiphyte communities exhibited responses to P addition that were most evident at the division level.
Habitat restoration can partially compensate for the extensive loss of coastal wetlands, but creation of functional habitat and assessment of restoration success remain challenging tasks. To evaluate wintering shorebird use of restored coastal wetlands, we quantified shorebird assemblages and behavior of selected focal species at five restored sites and paired reference sites in Mugu Lagoon, southern California, United States. The Shannon-Wiener index of species diversity (for all birds in order Charadriiformes) was higher in the restored than in the reference portion of three of the five sites, higher in the reference portion of a fourth site, and similar between reference and restored areas of the fifth site. Species diversity was lower in sites closer to man-made structures. The four most abundant species groups across the five sites were selected for detailed analysis of site use and behavior: Willets (Catoptrophorus semipalmatus), Marbled Godwits (Limosa fedoa), Dowitchers (Limnodromus spp.), and Sandpipers (Calidris spp.) (Western, Least, and Dunlin). Each focal species group exhibited distinct site preferences, and densities in restored sites were often as high or higher than in reference sites. Willets and Dowitchers preferred habitats with more extensive tidal flats, a characteristic of restored sites. Godwits and Sandpipers preferred heterogeneous habitats with a mix of water and tidal flats. Most birds were engaged in feeding activities during the ebb tides surveyed, and there were no apparent differences in behavior between reference and restored sites. Though not all restored sites were used equally by all species, the creation of multiple restored sites with varied habitat characteristics attracted a diverse assemblage of shorebirds and may have contributed to the integrity of the regional wetland landscape.
Establishing species-rich plant communities is a common goal of habitat restoration efforts, but not all species within a target assemblage have the same capacity for recruitment and survival in created habitats. We investigated the development of a tidal salt marsh plant community in the presence of a rapidly colonizing dominant species, Salicornia virginica, in a newly created habitat in Mugu Lagoon, California, USA. We planted rooted cuttings of S. virginica, Distichlis spicata, Jaumea carnosa, and Frankenia salina in single- and mixed-species stands, where each species was planted alone or in combination with S. virginica in 4 m 2 plots. We measured species percent cover, recruit density, canopy structure, and aboveground biomass after three growing seasons. When planted alone, S. virginica achieved the greatest cover, up to 70%, followed by J. carnosa (55%), F. salina (35%), and D. spicata (12%). Total percent cover was about 30% lower than in a reference site. For each species, average percent cover and aboveground biomass per plant were generally similar between single-species and mixed planting treatments, suggesting that on the time scale of this study, competition between species was weak. Canopy structure (height, number of layers) and total aboveground biomass of all species were largely unaffected by planting treatments, although S. virginica was shorter when planted with J. carnosa. Salicornia virginica recruits constituted approximately 98% of the cover of seedling recruits into the created site. Despite intense S. virginica recruitment, our intervention in the successional process by planting species with poorer colonization abilities, particularly J. carnosa and F. salina, prevented S. virginica from completely dominating the canopy, thus increasing vascular plant richness in the created site. Artificially increased richness may enhance some ecosystem functions and create a seed source to facilitate the persistence of a diverse plant assemblage in restored sites.
We evaluated how links between direct and indirect interactions and physical disturbance shaped trophic dynamics in a soft-sediment benthic estuarine community. We crossed presence of burrow-excavating crabs Pachygrapsus crassipes and nutrient enrichment (nitrogen and phosphorus) in cages containing herbivorous surface-feeding snails Cerithidea californica and benthic microalgae in a tidal mudflat and a tidal sandflat in Mugu Lagoon, southern California, USA. P. crassipes consumed up to 85 % of C. californica in enclosures, but there was no evidence of a trophic cascade, as crab reduction of snail density did not increase benthic microalgal biomass. Rather, P. crassipes decreased diatom and cyanobacterial biomass by up to 50 % in the mudflat and 80 % in the sandflat, probably via bioturbation. Subadult C. californica lengths increased 15 to 20 % over 5 wk in treatments without crabs. In the presence of P. crassipes, C. californica lengths increased < 5 %, probably an indirect result of crab reduction of microalgal food availability or increased snail burial. C. californica may have actively burrowed as an escape response from the crabs, or have been passively buried during crab burrowing activities. Nutrient addition did not reduce snail growth, but increased snail mortality at both sites, possibly a result of nutrient-induced shifts towards toxic or poor nutritive quality cyanobacteria. The top-down impacts of P. crassipes reduced the relative bottom-up effects of nutrients in this habitat, illustrating the importance of evaluating both biotic and abiotic interactions simultaneously. Numerous indirect and non-trophic interactions revealed a community structure that was much more complex than suggested by food web structure.
The coexistence of multiple species within atrophic level can be regulated by consumer preferences and nutrient supply, but the influence of these factors on the co-occurrence of seagrass species is not well understood. We examined the biomass and density responses of two seagrass species in the Florida Keys Reef Tract to grazing pressure near patch reefs, and evaluated how nutrient enrichment impacted herbivory dynamics. We transplanted Halodule wrightii (shoalgrass) sprigs into caged and uncaged plots in a Thalassia testudinum (turtlegrass) bed near a patch reef. Nutrients (N and P) were added to half of the experimental plots. We recorded changes in seagrass shoot density, and after three months, we measured above- and belowground biomass and tissue nutrient content of both species. Herbivory immediately and strongly impacted H. wrightii. Within six days of transplantation, herbivory reduced the density of uncaged H. wrightii by over 80%, resulting in a decrease in above- and belowground biomass of nearly an order of magnitude. T testudinum shoot density and belowground biomass were not affected by herbivory, but aboveground biomass and leaf surface area were higher within cages, suggesting that although herbivory influenced both seagrass species, T testudinum was more resistant to herbivory pressure than H. wrightii. Nutrient addition did not alter herbivory rates or the biomass of either species over the short-term duration of this study. In both species, nutrient addition had little effect on the tissue nutrient content of seagrass leaves, and N:P was near the 30:1 threshold that suggested a balance between N and P. The different impacts of grazing on these two seagrass species suggest that herbivory may be an important regulator of the distribution of multiple seagrass species near herbivore refuges like patch reefs in the Caribbean.
We evaluated how changes in nutrient supply altered the composition of epiphytic and benthic microalgal communities in a Thalassia testudinum (turtle grass) bed in Florida Bay. We established study plots at four sites in the bay and added nitrogen (N) and phosphorus (P) to the sediments in a factorial design. After 18, 24, and 30 months of fertilization we measured the pigment concentrations in the epiphytic and benthic microalgal assemblages using high performance liquid chromatography. Overall, the epiphytic assemblage was P-limited in the eastern portion of the bay, but each phototrophic group displayed unique spatial and temporal responses to N and P addition. Epiphytic chlorophyll a, an indicator of total microalgal load, and epiphytic fucoxanthin, an indicator of diatoms, increased in response to P addition at one eastern bay site, decreased at another eastern bay site, and were not affected by P or N addition at two western bay sites. Epiphytic zeaxanthin, an indicator of the cyanobacteria/coralline red algae complex, and epiphytic chlorophyll b, an indicator of green algae, generally increased in response to P addition at both eastern bay sites but did not respond to P or N addition in the western bay. Benthic chlorophyll a, chlorophyll b, fucoxanthin, and zeaxanthin showed complex responses to N and P addition in the eastern bay, suggesting that the benthic assemblage is limited by both N and P. Benthic assemblages in the western bay were variable over time and displayed few responses to N or P addition. The contrasting nutrient limitation patterns between the epiphytic and benthic communities in the eastern bay suggest that altering nutrient input to the bay, as might occur during Everglades restoration, can shift microalgal community structure, which may subsequently alter food web support for upper trophic levels.
The capacity of epifauna to control algal proliferation following nutrient input depends on responses of both grazers and upper trophic level consumers to enrichment. We examined the responses of Thalassia testudinum (turtle grass) epifaunal assemblages to nutrient enrichment at two sites in Florida Bay with varying levels of phosphorus limitation. We compared epifaunal density, biomass, and species diversity in 2 in 2 plots that had either ambient nutrient concentrations or had been enriched with nitrogen and phosphorus for 6 months. At the severely P-limited site, total epifaunal density and biomass were two times higher in enriched than in unenriched plots. Caridean shrimp, grazing isopods, and gammarid amphipods accounted for much of the increase in density; brachyuran crabs, primary predatory fish, and detritivorous sea cucumbers accounted for most of the increase in biomass. At the less P-limited site, total epifaunal density and biomass were not affected by nutrient addition, although there were more caridean shrimp and higher brachyuran crab and pink shrimp biomass in enriched plots. At both sites, some variation in epifaunal density and biomass was explained by features of the macrophyte canopy, such as T. testudinum and Halodule ivrightii percent cover, suggesting that enrichment may change the refuge value of the macrophyte canopy for epifauna. Additional variation in epifaunal density and biomass was explained by epiphyte pigment concentrations, suggesting that enrichment may change the microalgal food resources that support grazing epifauna. Increased epifaunal density in enriched plots suggests that grazers may be able to control epiphytic algal proliferation following moderate nutrient input to Florida Bay.
Armitage, A.R., T.A. Frankovich, K.L. Heck, Jr., and J.W. Fourqurean. 2005. Experimental nutrient enrichment causes complex changes in seagrass, microalgae, and macroalgae community structure in Florida Bay. Estuaries 28: 422-434.
We examined the spatial extent of nitrogen (N) and phosphorus (P) limitation of each of the major benthic primary producer groups in Florida Bay (seagrass, epiphytes, macroalgae, and benthic microalgae) and characterized the shifts in primary producer community composition following nutrient enrichment. We established 24 permanent 0.25-m(2) Study plots at each of six sites across Florida Bay and added N and P to the sediments in a factorial design for 18 mo. Tissue nutrient content of the turtlegrass Thalassia testudinum revealed a spatial pattern in P limitation, from severe limitation in the eastern bay (N:P > 96:1), moderate limitation in two intermediate sites (approximately 63:1), and balanced with N availability in the western bay (approximately 31: 1). P addition increased T. testudinum cover by 50-75% and short-shoot productivity by up to 100%, but only at the severely P-limited sites. At sites with an ambient N:P ratio suggesting moderate P limitation, few seagrass responses to nutrients occurred. Where ambient T. testudinum tissue N:P ratios indicated N and P availability was balanced, seagrass was not affected by nutrient addition but was strongly influenced by disturbance (currents, erosion). Macroalgal and epiphytic and benthic microalgal biomass were variable between sites and treatments. In general, there was no algal overgrowth of the seagrass in enriched conditions, possibly due to the strength of seasonal influences on algal biomass or regulation by grazers. N addition had little effect on any benthic primary producers throughout the bay. The Florida Bay benthic primary producer community was P limited, but P-induced alterations of community structure were not uniform among primary producers or across Florida Bay and did not always agree with expected patterns of nutrient limitation based on stoichiometric predictions from field assays of T. testudinum tissue N:P ratios.
We investigated the assumption that populations of epibenthic macroinvertebrates readily establish in created coastal wetlands by quantifying the spatial and temporal patterns of Cerithidea californica (California horn snail) density in a newly created wetland and an adjacent natural area in Mugu Lagoon, southern California, United States, for 3.5 years. The natural and created sites differed in vascular plant cover and sediment grain size, organic content, salinity, and moisture content. Densities of C. californica in the created site changed little during the study period, and were often lower than those in the natural site. The influences of habitat suitability and dispersal limitation on C. californica colonization of the created site varied among snail age classes. Sediment moisture and organic content explained some of the variability in subadult (47%) and adult (55%) density and relative abundance, but none of the variability in juvenile abundance. Adult snail density was also strongly influenced by distance from the natural/created site transition zone. Juvenile and subadult snail densities were not related to distance from the natural site, possibly due to greater dispersal ability. Between-site differences in C. californica densities and size structure suggested that adult snails were affected by both habitat characteristics and dispersal ability, subadults were influenced to a lesser degree by habitat characteristics, and juveniles were not related to either factor. Accordingly, the influence of habitat characteristics and dispersal ability on created site colonization may change with snail age. Successful restoration of benthic invertebrate communities requires consideration of both habitat characteristics and dispersal ability of the target species, even in created sites in close proximity to natural areas.
We evaluated the effects of nutrient addition on interactions between the benthic microalgal community and a dominant herbivorous gastropod, Cerithidea californica (California horn snail), on tidal flats in Mugu Lagoon, southern California, USA. We crossed snail and nutrient (N and P) addition treatments in enclosures on two tidal flats varying from 71 to 92% sand content in a temporally replicated experiment (summer 2000, fall 2000, spring 2001). Diatom biomass increased slightly (similar to30%) in response to nutrient treatments but was not affected by snails. Blooms of cyanobacteria (up to 200%) and purple sulfur bacteria (up to 400%) occurred in response to nutrient enrichment, particularly in the sandier site, but only cyanobacterial biomass decreased in response to snail grazing. Snail mortality was 2-5 times higher in response to nutrient addition, especially in the sandier site, corresponding to a relative increase in cyanobacterial biomass. Nutrient-related snail mortality occurred only in the spring and summer, when the snails were most actively feeding on the microalgal community. Inactive snails in the fall showed no response to nutrient-induced cyanobacterial growths. This study demonstrated strongly negative upward cascading effects of nutrient enrichment through the food chain. The strength of this upward cascade was closely linked to sediment type and microalgal community composition.
Boyer, K.E., P. Fong, A.R. Armitage, and R.A. Cohen. 2004. Elevated nutrient content of tropical macroalgae increases rates of herbivory in coral, seagrass, and mangrove habitats. Coral Reefs 23:530-538.
We explored the role of food quality in herbivore preference for macroalgae by comparing consumption of Acanthophora spicifera with and without elevated tissue nitrogen and phosphorus concentrations. Algal enrichment effects on herbivory were examined in coral, seagrass, and mangrove habitats along a sparsely populated Honduran island protected from fishing. Nutrient enrichment led to significantly increased grazing by herbivores across habitats. Consumption of enriched algae increased by 91% compared to controls among the mangrove roots, where herbivory rates were generally lowest. In the heavily grazed seagrass and coral habitats, nutrient enrichment increased consumption by 30 and 20%, respectively, with the effect more spatially variable than among the mangrove roots. We suggest that, at least on the local scale, intact herbivore populations may be able to compensate for effects of increased nutrient supply by locating and consuming nutrient-enriched algae, but that the importance of this mechanism varies both among and within habitats.