Anna R. Armitage

Experimental nutrient enrichment causes complex changes in seagrass, microalgae, and macroalgae community structure in florida bay

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-m2 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 turtlegrassThalassia 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 increasedT. 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 ambientT. 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 ofT. testudinum tissue, N:P ratios.

Elevated nutrient content of tropical macroalgae increases rates of herbivory in coral, seagrass, and mangrove habitats

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.

Upward cascading effects of nutrients: shifts in a benthic microalgal community and a negative herbivore response

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 (~30%) 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.

The contribution of mangrove expansion to salt marsh loss on the Texas Gulf Coast.

Landscape-level shifts in plant species distribution and abundance can fundamentally change the ecology of an ecosystem. Such shifts are occurring within mangrove-marsh ecotones, where over the last few decades, relatively mild winters have led to mangrove expansion into areas previously occupied by salt marsh plants. On the Texas (USA) coast of the western Gulf of Mexico, most cases of mangrove expansion have been documented within specific bays or watersheds. Based on this body of relatively small-scale work and broader global patterns of mangrove expansion, we hypothesized that there has been a recent regional-level displacement of salt marshes by mangroves. We classified Landsat-5 Thematic Mapper images using artificial neural networks to quantify black mangrove (Avicennia germinans) expansion and salt marsh (Spartina alterniflora and other grass and forb species) loss over 20 years across the entire Texas coast. Between 1990 and 2010, mangrove area grew by 16.1 km2, a 74% increase. Concurrently, salt marsh area decreased by 77.8 km2, a 24% net loss. Only 6% of that loss was attributable to mangrove expansion; most salt marsh was lost due to conversion to tidal flats or water, likely a result of relative sea level rise. Our research confirmed that mangroves are expanding and, in some instances, displacing salt marshes at certain locations. However, this shift is not widespread when analyzed at a larger, regional level. Rather, local, relative sea level rise was indirectly implicated as another important driver causing regional-level salt marsh loss. Climate change is expected to accelerate both sea level rise and mangrove expansion; these mechanisms are likely to interact synergistically and contribute to salt marsh loss.

Nutrient effects on seagrass epiphyte community structure in Florida Bay.

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.

RESTORING ASSEMBLAGES OF SALT MARSH HALOPHYTES IN THE PRESENCE OF A RAPIDLY COLONIZING DOMINANT SPECIES

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 m2 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.

Evaluating Salt Marsh Restoration Success with an Index of Ecosystem Integrity

ABSTRACT Staszak, L.A. and Armitage, A.R., 2013. Evaluating salt marsh restoration success with an index of ecosystem integrity. 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 (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 y) and reference marshes. Thirteen biotic and abiotic characteristics were used to calculate an ecosystem index score, whereby a 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.

Avicennia germinans (black mangrove) vessel architecture is linked to chilling and salinity tolerance in the Gulf of Mexico.

Over the last several decades, the distribution of the black mangrove Avicennia germinans in the Gulf of Mexico has expanded, in part because it can survive the occasional freeze events and high soil salinities characteristic of the area. Vessel architecture may influence mangrove chilling and salinity tolerance. We surveyed populations of A. germinans throughout the Gulf to determine if vessel architecture was linked to field environmental conditions. We measured vessel density, hydraulically weighted vessel diameter, potential conductance capacity, and maximum tensile fracture stress. At each sampling site we recorded mangrove canopy height and soil salinity, and determined average minimum winter temperature from archived weather records. At a subset of sites, we measured carbon fixation rates using a LI-COR 6400XT Portable Photosynthesis System. Populations of A. germinans from cooler areas (Texas and Louisiana) had narrower vessels, likely reducing the risk of freeze-induced embolisms but also decreasing water conductance capacity. Vessels were also narrower in regions with high soil salinity, including Texas, USA and tidal flats in Veracruz, Mexico. Vessel density did not consistently vary with temperature or soil salinity. In abiotically stressful areas, A. germinans had a safe hydraulic architecture with narrower vessels that may increase local survival. This safe architecture appears to come at a substantial physiological cost in terms of reduction in conductance capacity and carbon fixation potential, likely contributing to lower canopy heights. The current distribution of A. germinans in the Gulf is influenced by the complex interplay between temperature, salinity, and vessel architecture. Given the plasticity of A. germinans vessel characters, it is likely that this mangrove species will be able to adapt to a wide range of potential future environmental conditions, and continue its expansion in the Gulf of Mexico in response to near-term climate change.

The Response of Photosystem II to Soil Salinity and Nutrients in Wetland Plant Species of the Northwestern Gulf of Mexico

Abstract Madrid, E.N.; Armitage, A.R., and Quigg, A., 2012. The response of photosystem II to soil salinity and nutrients in wetland plant species of the Northwestern Gulf of Mexico. 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 Spartina alterniflora (Loisel), Spartina patens ((Aiton) Muhl), Schoenoplectus californicus ((C.A. Mey.) Steud.) and Schoenoplectus robustus ((Pursh) M.T. Strong), common in brackish and salt marshes of the northwestern Gulf of Mexico. 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. Schoenoplectus 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 the species with the highest potential for photosynthetic performance (S. robustus and S. californicus) were only found in locations with favorable abiotic conditions. 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.

The Interactive Effects of Pulsed Grazing Disturbance and Patch Size Vary among Wetland Arthropod Guilds

Pulse disturbances and habitat patch size can determine community composition independently or in concert, and may be particularly influential on small spatial scales for organisms with low mobility. In a field experiment, we investigated whether the effects of a pulsed disturbance that simulated a grazing event varied with habitat patch size. We focused on the short-term responses of multiple co-occurring emergent salt marsh arthropods with differing levels of mobility and dispersal potential. As part of a marsh restoration project, two types of emergent marsh structures were created: small circular mounds (0.5 m diameter) separated by several meters of aquatic habitat, and larger, elongated terraces (>50 m long). Study plots (0.25 m2) were established on both structures; in a subset of plots, we simulated a pulsed grazing disturbance event by clipping the aboveground tissue of emergent plants, primarily Spartina alterniflora. At the end of the two-month recovery period, Ischnodemus (Hemiptera: Blissidae) density was over 50% lower in disturbed treatments within both large (terrace) and small (mound) patches. Predatory spider treatment responses were similar to Ischnodemus responses, suggesting a trophic relationship between those two arthropod groups. Alternatively, spiders may have been directly affected by the loss of shelter in the disturbed plots. Prokelisia (Homoptera: Delphacidae), which are generally more mobile than Ischnodemus, were not affected by disturbance treatment or by patch size, suggesting the potential for rapid recolonization following disturbance. Larval stem borers decreased by an order of magnitude in disturbed plots, but only in the large patches. In general, the disturbance effects of vegetation removal on arthropod density and community composition were stronger than patch size effects, and there were few interactions between pulsed disturbance and patch size. Rather, emergent marsh arthropod responses to disturbance and habitat area treatments were linked to the dispersal potential and mobility of each individual taxon.