No description available.

Salt marsh food webs are complex systems, with high levels of in situ primary production supporting a wealth of resident and migratory species. In this study, we use stable isotopes as a tool to trace organic matter utilization within the Nueces Marsh food web. Specifically, we were interested in (1) the use of treated wastewater as a ¹⁵N tracer, and (2) seasonal and interannual variation in food web structure. We hypothesized that treated wastewater would selectively label detrital resourse use within the food web, allowing us to trace grazing vs. detrital pathways within the marsh system. We also hypothesized that marsh consumers would exhibit distinct differences in isotopic composition between summer and winter, and between different years. We found that the Nueces Marsh food web consists of 3.5 consumer trophic levels. The [delta]¹³C values of consumer organisms were similar across the spatial extent of the low marsh, regardless of proximity to wastewater inflow. However, a majority of the organisms collected from the wastewater channel were significantly (p<0.05) enriched in ¹⁵N compared to their reference counterparts. We propose that ¹⁵N-enriched nitrogen is entering the Nueces Marsh food web through detrital rather than grazing-based pathways, making wastewater effluent an effective tracer of detrital integration into a marsh food web system. Hydrologic data indicate that isotopic shifts between seasons and between years reflected larger scale shifts between drought and wet years. During drought years, decreased production by phytoplankton and emergent plants led consumers to rely more heavily on ¹³C-enriched cyanobacterial carbon. In contrast, wet years encourage phytoplankton and emergent plant production, making cyanobacterial carbon relatively less exploited. While the Nueces Marsh food web is supported by a stable detrital carbon pool, it may still be susceptible to larger scale hydrologic events. / text

Thesis (M.S.)--Humboldt State University, 2007. / Includes bibliographical references (leaves 116-119). Also available via Humboldt Digital Scholar.

Thesis (Ph. D.)--M.I.T., Dept. of Biology, 1979. / Supervised by John M. Teal. Vita. Includes bibliographies.

Thesis (M.S.) in Marine Biology--University of Maine, 2006. / Includes vita. Includes bibliographical references (leaves 77-82).

Thesis (M.Sc.)--College of William and Mary. / Includes bibliographical references.

Ecophysiology studies are needed to predict plant responses in relation to climate change. Variations in salinity and inundation are expected to influence the survival and distribution of salt marsh. The following species were chosen to study as they occur in most South African salt marshes and are representative of different tidal ranges; namely Triglochin buchenaui (lower intertidal), Bassia diffusa (upper intertidal succulent) and Limonium linifolium (upper intertidal non-succulent). To simulate climate change conditions as predicted for South Africa, a 3 x 5 controlled experiment of three inundation levels (tidal, submerged and drought) and five salinity levels (0, 8, 18, 35, 45 ppt) was conducted for each species. This resulted in 15 treatments per species. Plant responses were measured over a three month period. Triglochin buchenaui showed significant variation in height (7.57 ± 0.5 to 29 ± 1.55 cm, p < 0.005, DF = 55) with optimum growth at 0 ppt under tidal conditions; leaf area increments and relative growth rates which decreased with increasing salinity under all inundation states. There was almost a cessation of growth under submergence which reduces the plant’s regeneration potential under these conditions. Proline accumulation (1.84 ± 0.23 to 3.36 ± 0.38 mg l-1), response of photosynthetic pigments and electrolyte leakage (8.17 ± 0.80 to 38.36 ± 7.42 percent) were fundamental to osmotic and membrane response regulation. Plants survived in all inundation states at salinity up to 45 ppt, but the optimum range was 0 to 18 ppt, and best water state was the tidal condition. Viable rhizomes were produced under drought conditions. Bassia diffusa (Thunb.) Kuntze plants under submergence died within one month, irrespective of the salinity. Optimum growth occurred in plants of the tidal treatment at 18 ppt, and reduced with increased salinity and drought conditions. Plants in the tidal treatments were more succulent than the drought-treated plants. There was reduced leaf mass and high anthocyanin concentrations in drought-treated plants and these effects increased with salinity. Soil and leaf water potential were positively correlated with anthocyanin concentration in leaves and stems, suggesting anthocyanin accumulated in response to drought and could be an adaptation to lower the plant’s water potential under drought conditions. A shift of anthocyanin from leaves to stems was found in drought-treated plants, and this possibly enables the maximization of photosynthesis in leaves, to complement its role in osmotic balance and photo-protection. Growth of Limonium linifolium showed that the plant was tolerant to a wide range of salinity under both tidal and drought conditions, but was susceptible to complete submergence, with high membrane damage even in tidal-treated plants. Plants died within 2 weeks of complete submergence. Results further indicated that L. linifolium tolerates extreme drought by accumulating large quantities of proline and oxalic acid, which consequently lowers its water potential for uptake of soil water of high salinity. Excess salts were excreted through salt glands. This is an important adaptation for a plant that thrives in a highly variable saline habitat Further investigation of submergence effects on upper intertidal species using B. diffusa showed three key stages in the response. A drop in chlorophyll a+b within 6 hours (4.2 ± 0.2 to 2.4 ± 0.3 mg l-1) with a corresponding increase in carotenoid concentration (0.6 ± 0.1 mg l-1) indicated an immediate response to submergence. Oxalic acid concentration was highest on Day 4 (13.6 mM) as opposed to control levels, indicative of its role in submergence tolerance, thus Day 4 may be the peak of positive acclimation. The third phase was marked by a sharp increase in electrolyte leakage to 47.5 ± 2.6 percent on Day 10, from 9.4 ± 1.4 percent on Day 7, with a corresponding decrease in total dissolved solutes between Days 7 and 10. Results suggest that oxalic acid accumulates under submergence possibly as a stabilizing osmolyte. The threshold for tolerance of the species under submergence is 7 days with membrane damage thereafter. Bassia diffusa would not survive prolonged submergence (> 7 days) but could survive submergence of short duration (< 7 days) through continuous underwater photosynthesis, accumulation of osmolytes such as oxalic acid and carotenoid, and maintenance of relative water content and succulence within control levels. When considered together, results showed that the two upper intertidal species were sensitive to waterlogging and would not survive complete submergence, whereas the lower intertidal species could in addition to its natural range, thrive in conditions typical of the upper intertidal range, namely prolonged dry conditions and high sediment salinity. These results have important implications for the future management of salt marshes under predicted climate change conditions. In permanently open South African estuaries, a landward migration of salt marsh will be possible if coastal squeeze is limited and the rate of landward recruitment is on par with sea level rise. In this case salt marsh species would retain their current zonation while shifting inland. Increased sea storms and saltwater intrusion could lead to high salinity concentration in the sediment and significantly reduce growth of salt-sensitive plants. In estuaries that are temporarily open to the sea, reduced freshwater inflow will result in an increase in mouth closure, high water levels, prolonged submergence, and consequently die back of salt marsh vegetation. On the other hand increased abstraction and drought would result in low water levels and high sediment salinity which would decrease growth and survival of salt marsh. This research has provided new knowledge on the ecophysiology of salt marsh plants which can be used to predict the responses of plants to climate change.

No description available.

The low-lying topographic nature of salt marshes makes plants in these communities particularly vulnerable to increased salinity and inundation exposure associated with sea level rise. Both increased salinity and inundation have been cited as major causes of reduced plant performance and survival in marsh and areas fringing marsh. In addition to limitations imposed by physical stress, interspecific interactions have also been shown to mediate the performance and survival of salt marsh and salt marsh fringing species. The Stress Gradient Hypothesis (SGH) postulates that species interactions shift from competitive to facilitative as stress levels increase and predicts that (a) the frequency and intensity of facilitative interactions increase as conditions become more stressful for plants and (b) the strength of competitive interactions increases as abiotic stress levels diminish. The SGH has been rigorously tested to examine how both the frequency and intensity of species interactions change under varying physical stress levels. Studies conducted in salt marsh systems have shown facilitation to be as strong of a driving force as competition in influencing plant performance and survival and have shown that while competition appears to be the pervasive force in the less physically stressful terrestrial zones fringing salt marshes, facilitation influences the performance and survival of species in harsher marsh areas. Under conditions of sea level rise, it remains unclear if the nature of interspecific interactions would shift as stress levels change. This research endeavors to examine the interplay between abiotic stresses and biotic interactions under conditions of increased salinity and inundation exposure. The first study presented here investigated the effects of increased inundation and soil salinity associated with sea level rise on four salt marsh fringing species, and assesses how competition and facilitation impact survival of salt marsh fringing plant survival under these changing conditions. All plant species experienced reduced growth and photosynthetic inhibition below their current distributional positions, both in the presence and absence of neighboring above ground vegetation. The findings also signal a potential shift in the nature of interspecific interactions from competition to facilitation to neutral as plants begin to experience increased salt and inundation exposure. The second study aimed to disentangle the effects of increased soil salinity and increased soil moisture on four salt marsh fringing species, and to examine the effects of plant neighbors. The results showed that fringe plants exposed to increased inundation experienced a two-fold reduction in performance and survival over 750 g pure salt addition, suggesting that inundation may be a more important limiting factor than salinity with rising sea levels. Landward transplants at the forest-fringe margin exposed to lower soil salinity and decreased inundation exhibited a three-fold increase in performance and survival when compared to controls. Neighbor manipulation studies, which consisted of trimming neighboring vegetation to ground level, again suggested that interspecific interactions in salt marsh fringing species may shift from competitive to facilitative with climate-induced sea level rise. Overall, our findings suggest that salt marsh fringing species may not be able to tolerate changing conditions associated with sea level rise and their survival may hinge on their ability to migrate towards higher elevations. The final experiment tested the Stress Gradient Hypothesis and investigated the relative importance of facilitation and competition in a salt marsh system under varying stress levels. This study also ascertained whether salt or inundation exposure is the primary influence on salt marsh plant performance and survival. As in previous studies, our findings suggest that many salt marsh plants don't require, but merely tolerate harsher abiotic conditions. The results showed that plants at higher elevations were depressed by strong competitive pressure from neighboring fringe species while plants at lower elevations benefited from the presence of neighbors. Collectively, the results of these studies indicate that species interactions are an integral driver of plant distribution in salt marsh communities. Furthermore, our findings indicate that changing stress levels may not always result in a shift in the nature of interspecific interactions. These studies have endeavored to show that the interplay between competition and facilitation interacts with physical processes to determine the growth and performance of both fringe and marsh plant species. The paucity of studies examining the roles of species interactions and changing abiotic stress levels on multiple salt marsh and salt marsh fringing species warrants the need for additional research. The responses of salt marsh and salt marsh fringing species to sea level rise can not only serve as very valuable and sensitive indictors of climate change, but will also aid in predicting the future location of the marsh-fringe-forest ecotone, which is predicted to shift inland as sea levels continue to rise.

Thesis (Ph.D.)--University of North Carolina at Wilmington, 2006. / Includes bibliographical references (leaves: 198-208)