Keith Walters

Interactions between multiple recruitment drivers: post-settlement predation mortality and flow-mediated recruitment.

Background Dispersal is a primary driver in shaping the future distribution of species in both terrestrial and marine systems. Physical transport by advection can regulate the distance travelled and rate of propagule supply to a habitat but post-settlement processes such as predation can decouple supply from recruitment. The effect of flow-mediated recruitment and predation on the recruitment success of an intertidal species, the eastern oyster Crassostrea virginica was evaluated in two-replicated field experiments. Two key crab species were manipulated to test predator identity effects on oyster mortality. Findings Recruitment was ∼58% higher in high flow compared to low flow, but predation masked those differences. Predation mortality was primarily attributed to the blue crab Callinectes sapidus, whilst the mud crab Panopeus herbstii had no effect on recruit mortality. Recruit mortality from predation was high when recruit densities were high, but when recruit density was low, predation effects were not seen. Under high recruitment (supply), predation determined maximum population size and in low flow environments, recruitment success is likely determined by a combination of recruitment and resource limitation but not predation. Conclusions Four processes are demonstrated: (1) Increases in flow rate positively affect recruitment success; (2) In high flow (recruitment) environments, resource availability is less important than predation; (3) predation is an important source of recruit mortality, but is dependent upon recruit density; and (4) recruitment and/or resource limitation is likely a major driver of population structure and functioning, modifying the interaction between predators and prey. Simultaneous testing of flow-mediated recruitment and predation was required to differentiate between the role of each process in determining population size. Our results reinforce the importance of propagule pressure, predation and post-settlement mortality as important determinants of population growth and persistence, but demonstrate that they should not be considered mutually exclusive.

GASTROPOD ABUNDANCE AND BIOMASS RELATIONSHIPS WITH SALT MARSH VEGETATION WITHIN OCEAN-DOMINATED SOUTH CAROLINA, USA ESTUARIES

Abstract Molluscs, both gastropods and bivalves, appear to disproportionately influence the dynamics of salt marsh habitats. Bivalves typically are facilitators positively affecting the growth and survival of marshes, but gastropods may have unanticipated top-down effects through direct consumption of the predominant marsh vegetation, Spartina alterniflora. Natural, elevational differences in S. alterniflora density and morphology also may exert an influence on the spatial distributions of marsh snails. We examined the abundance of both plants and snails across an elevation gradient to determine if similarities or differences existed in the observed distribution patterns. Plant, mainly S. alterniflora, and snail, Littoraria irrorata (Say) and Melampus bidentatus Say, densities were measured along tidal creek to forest transects within the intertidal marshes of three marine-dominated South Carolina inlets. Significant differences in S. alterniflora density among marsh zones were paralleled by similar L. irrorata density differences. A consistent, unimodal pattern with peak L. irrorata density in the “short,” dense S. alterniflora mid marsh occurred within each inlet. In contrast, M. bidentatus was restricted almost entirely to the high marsh across all transects and inlets. Densities of L. irrorata and S. alterniflora were positively correlated in the low marsh consistent with the potential importance of culms as a food source and refuge from predators. Littoraria irrorata biomass was significantly different among zones in only one inlet and increased in a shoreward direction in two inlets. Although biomass patterns previously have not been reported for Southeastern US marshes, amounts were consistently greater than expected within the high-marsh zone. In SC, L. irrorata appears to be a conspicuous resident across the entire vegetated marsh and our results suggest previous studies focusing on the marsh near tidal creeks could underestimate appreciably the total population and overall significance of L. irrorata within coastal marshes.

Within- and between-plant distribution of harpacticoid copepods in a North Atlantic bed of Laminaria ochroleuca

The fauna of harpacticoid copepods associated with a North Atlantic sublittoral population of Laminaria ochroleuca in Spain was investigated. Copepods belonging to four different orders, i.e. Harpacticoida, Cyclopoida, Poecilostomatoida and Siphonostomatoida, most of which were harpacticoids belonging to 17 different families were collected. Because the family Thalestridae was particularly abundant in the samples, its members were identified down to the species level, totalling 20 species. The copepod fauna was richer and more diverse on holdfasts than on fronds, irrespective of considering the entire harpacticoid assemblage at the family level or just the family Thalestridae at the species level. The effect of these two within-plant microhabitats on the taxonomic composition of the copepod fauna was substantial, with faunal similarity between fronds of plants from different sampling quadrats being usually higher than that between the frond and the holdfast of the same plant. Thalestridae, Harpacticidae and Tisbidae predominated on the fronds, while Diosaccidae, Ameiridae and Ectinosomatidae predominated on the holdfasts. Among the thalestrids, Parathalestris clausi, Paradactylopodia sp.l, Dactylopusia tisboides, and Dactylopusia vulgaris were common species, with Parathalestris clausi being the most abundant copepod on both fronds and holdfasts. In summary, the fauna of the holdfasts consists mostly of common, eurytopic species, while the fauna of the fronds comprises rare taxa-at least within the scope of the study-with low total abundances and/or usually occurring in few plants.

Effects of Shoreline Development on Composition and Physical Structure of Plants in a South Carolina High Marsh

Increased freshwater and nutrient runoff associated with coastal development is implicated in dramatically altering estuarine communities along eastern US shorelines. We examined effects of three categories of shoreline development on high-marsh environments within Murrells Inlet, South Carolina, USA by measuring sediment nutrients, porewater salinity, plant species diversity, and above- and belowground plant biomass. Effects on new plant growth also were examined in plot clearing and transplantation experiments. Greater nutrient availability in sediments along developed shorelines was reflected in greater aboveground biomass and nitrogen storage in Juncus roemerianus plant tissue. Plant species composition was not significantly different among levels of shoreline development. Zinc concentrations were greater in sediments from developed shorelines and may represent an easily measured indicator of shoreline development. Recently accelerating shoreline development in the southeastern USA may alter plant production, nitrogen storage, and sediment metal content in salt marshes.

Local-scale characteristics of high-marsh communities next to developed and undeveloped shorelines in an ocean-dominated estuary, Murrells Inlet, SC

Anthropogenic alteration of terrestrial shorelines can have pronounced effects on marine environments at the upland-marsh boundary. Possible terrestrial development effects on several physical and biological variables of high-marsh habitats were examined along developed and undeveloped shorelines in an ocean-dominated, southeastern US estuary. Analyses of sediment characteristics suggested development of the upland boundary affected physical conditions within the high-marsh. For example, pore water salinities were greater along undeveloped shorelines during a non-drought period even after rain events. Significant floral and faunal differences also existed between shoreline treatments. Black needle rush stems were significantly taller and marsh periwinkle densities significantly greater, but eastern coffee bean snail densities were significantly reduced along developed shorelines. Benthic infaunal community abundance and composition also were significantly different between shoreline treatments with sand fly larvae, human pest precursors, either only present or present in greater densities along developed shorelines. Sediment respirometry experiments indicated significant differences in heterotrophic and autotrophic processes occurring between shoreline treatments. Greater sediment surface temperatures along developed shorelines provided one possible mechanism driving high-marsh responses to boundary alteration. The history and extent of shoreline development along with a tendency in ocean-dominated southeastern marshes to resist change likely influenced current ecological conditions within our high-marsh study areas. A greater understanding of the driving mechanisms producing localized effects on salt marshes and recognizing regional differences in marsh resistance to change will facilitate predictions of shoreline development consequences and help in proposing effective management strategies for coastal boundaries.

Management of Plant Invaders Within a Marsh: An Organizing Principle for Ecological Restoration?

Controlling plant invaders is often one aspect of ecological restoration. However, the planning and application of control measures can lead to difficult questions regarding project goals and measures of success. We present a case study of a coastal wetland system in South Carolina, USA, where two plant invaders, Phragmites australisand Typha domingensis,were targeted for control. As project participants gradually accepted the concept that success must be measured in terms of long-term system parameters rather than short-term invader control, the methods and approaches changed. As an alternative to applying herbicides, a method of reconnecting the system to the ocean was pursued. Instead of simply measuring plant control, a before-after-control-impact monitoring design was implemented that allowed comparison among restored and multiple reference systems in the immediate area. Attempts to reestablish tidal flow and modify environmental conditions to alter system attributes were variable with both unplanned positive and negative effects. Most of these impacts were associated with the fact that the wetland existed in a state park used by large numbers of people for passive recreation. The case study demonstrates that plant invasion and the willingness of people to control plant invaders can provide a useful starting point for eventual development and implementation of scientifically meaningful attempts at ecological restoration.