Martin H. Posey

Community Changes Associated with the Spread of an Introduced Seagrass, Zostera Japonica

Species introductions have provided a valuable source of information for understanding the factors that regulate community composition. However, the effect of such introductions has often been obscured by a lack of information on distribution and abundance patterns before or during an invasion event. I examined the changes in a benthic community associated with the ongoing spread of an introduced seagrass, Zosterajaponica, by sampling transplanted seagrass plots and established Z. japonica patches of known ages. The sedimentary and faunal changes associated with Z. japonica were similar to those observed with native seagrass species. Mean sediment grain size declined and sediment volatile organics increased within Z. japonica patches. Faunal richness was higher within Z. japonica patches compared with adjacent unvegetated areas, and many numerically dominant species were positively associated with this introduced seagrass. However, the effects of Z. japonica on faunal abundance varied with both the age of a seagrass patch and site location. The introduction of this seagrass has thus changed the physical habitat as well as the richness and densities of resident fauna. Many studies of introduced species have concentrated on direct interactions between introduced and native organisms. In contrast, the community changes associated with the introduction of Z. japonica emphasize the potential importance of indirect and system- level effects of introduced species on community composition.

Complex predator-prey interactions within an estuarine benthic community

Indirect predator—prey interactions have been suggested to be important in a variety of communities. However, we still understand little about the factors determining the relative importance of indirect and direct effects of predation or what forms indirect predator—prey interactions may take. We examined the direct effects of predation by an omnivorous grass shrimp. Palaemonetes pugio, and the indirect effects for prey species of interactions between this shrimp and other predators in an estuarine benthic community. Direct and indirect effects of predation were examined in the field using a combination of large— and small—scale enclosure/exclosure experiments. Specific interactions and mechanisms of effects were examined in laboratory predation studies, utilizing both two—species and multi—species systems. Grass shrimp predation reduced the densities of a variety of benthic fauna, but the effects varied seasonally and with prey size. Interactions between grass shrimp and other predators took two forms: reduction of an infaunal predator by shrimp predation and predation on grass shrimp by fish. Both types of interactions had a mixed effect on lower trophic levels, with enhancement of certain benthic prey and no effect on other species. Interactions between grass shrimp and their fish predators resulted in changes in shrimp distribution independent of changes in shrimp abundance, creating spatial refungia for shrimp prey. The dynamics of indirect interactions in this community are strongly affected by the omnivorous nature of the major predators. Our results emphasize the general importance of omnivory in regulating predator—prey interactions and in predicting the relative importance of indirect predator effects.

Organism responses to habitat fragmentation and diversity: Habitat colonization by estuarine macrofauna

Abstract Ecologists increasingly recognize that their choice of spatial scales may influence greatly their interpretation of ecological systems, and that small changes in the patchiness of habitat resources can produce abrupt, sometimes dramatic shifts in distribution and abundance patterns of a species. Moreover, identification of scale- and habitat-dependent ecological patterns are central to management efforts aimed at predicting the response of organisms to the increasing threat of habitat fragmentation. We used habitat plots containing artificial seagrass, oyster shell, and a mixture of seagrass and shell, placed on unstructured seafloor for 14 days in Back Sound, North Carolina, USA to examine the interactive effects of patch size, habitat diversity and experimental site on colonization by assemblages of estuarine macrofauna. We tested three a priori predictions of the general hypothesis that macrofaunal colonization is scale- and habitat-dependent: (1) colonization (per unit area) will be higher in small patches than in large ones; (2) small macrofauna will show a stronger response to habitat patchiness at a given scale than large macrofauna; and (3) colonization by estuarine macrofauna will be higher in habitat plots containing a mixture of seagrass and oyster shell compared to monotypic plots. Macrofauna responded to habitat patchiness in a complex manner that varied according to habitat type, experimental site, species, taxon, functional group, and animal body size (small: 500 μm–2 mm; large: >2 mm). Of the five out of seven response variables where we observed a significant patch size effect, grass shrimp ( Palaemonidae sp.) and small, mobile crustaceans (i.e., amphipods and isopods) were the only taxonomic or functional groups whose densities were higher in small (0.25 m 2 ) than large (1 m 2 ) patches, as predicted. Moreover, there was a disproportionate reduction in macrofaunal abundance and diversity in small patches of oyster shell compared to seagrass and mixed habitat treatments; this pattern was significant for both the total density and numbers of small species but not for large macrofauna. The total density and number of macrofaunal species was not higher in the mixed habitat treatment compared to seagrass or oyster shell. Our study demonstrates that an organisms response to habitat patchiness is dependent upon species, taxa, functional group, and animal body size, and that an organisms response is further modified by habitat type. The patterns observed in this study highlight the importance of scale- and habitat-dependent responses by mobile organisms to complex benthic habitats, and, because of the disproportionate reduction in faunal density and diversity in small versus large patches of oyster shell, heightens concern over the negative impacts to biodiversity through large-scale fragmentation of subtidal oyster reefs in certain regions.

Effects of a burrowing mud shrimp, Upogebia pugettensis (Dana), on abundances of macro-infauna

Abstract Burrowing ghost shrimp (Callianassidae) have been shown to exert a strong influence on the abundance of macro-infauna in soft-sediment communities, especially through reduction in the densities of many sedentary species. However, relatively little is known about the community effects of other burrowing shrimp, such as mud shrimp, Upogebia spp., that occur commonly in many estuaries of North America. A transplant/exclusion experiment was conducted to examine the effects of Upogebia pugettensis (Dana) on the abundance of other macro-infauna in a Pacific Northwest tideflat. Several numerically dominant, sedentary crustaceans and polychaetes exhibited lower abundances in the presence of Upogebia as compared to treatments where the mud shrimp was absent. The overall pattern of faunal response to the presence of U. pugettensis was similar to that reported for Callianassa californiensis (Dana), an unexpected finding given the different burrowing and feeding habits of these two crustaceans.

HURRICANE EFFECTS ON WATER QUALITY AND BENTHOS IN THE CAPE FEAR WATERSHED: NATURAL AND ANTHROPOGENIC IMPACTS

In the summer of 1996, southeastern North Carolina, United States, was struck by two hurricanes, with the second (Hurricane Fran) doing considerably more damage than the first (Hurricane Bertha). The Cape Fear watershed, largest in North Carolina, suffered from severe water quality problems for weeks following Fran, including a massive fish kill in the Northeast Cape Fear River. Post-hurricane flooding caused inputs of riparian swamp water to river channels, and sewage treatment plant and pump station power failures caused diversions of millions of liters of raw and partially treated human waste into rivers. Additionally, several swine waste lagoons were breached, overtopped, or inundated, discharging large quantities of concentrated organic waste into the system, particularly into the Northeast Cape Fear River. Dissolved oxygen (DO) decreased to 2 mg/L in the mainstem Cape Fear River, and fell to zero in the Northeast Cape Fear River for >3 wk. Biochemical oxygen demand in the Northeast Cape Fear River w...

North and South Carolina coasts

Abstract This coastal region of North and South Carolina is a gently sloping plain, containing large riverine estuaries, sounds, lagoons, and salt marshes. The most striking feature is the large, enclosed sound known as the Albemarle–Pamlico Estuarine System, covering approximately 7530 km 2 . The coast also has numerous tidal creek estuaries ranging from 1 to 10 km in length. This coast has a rapidly growing population and greatly increasing point and non-point sources of pollution. Agriculture is important to the region, swine rearing notably increasing fourfold during the 1990s. Estuarine phytoplankton communities in North Carolina are well studied; the most important taxonomic groups are diatoms, dinoflagellates, cryptomonads and cyanobacteria. Several major poorly flushed estuaries are eutrophic due to nutrient inputs, and toxic dinoflagellates ( Pfiesteria spp) can reach high densities in nutrient-enriched areas. Fully marine waters are relatively oligotrophic. Southern species enter in subsurface intrusions, eddies, and occasional Gulf Stream rings, while cool water species enter with the flow of the Labrador Current to the Cape Hatteras region. The Carolinas have a low number of endemic macroalgae, but species diversity can be high in this transitional area, which represents the southernmost extension for some cold-adapted species and the northernmost extension of warm-adapted species. In North Carolina the dominant seagrass, Zostera marina , lies at its southernmost extension, while a second species, Halodule wrightii is at its northernmost extent. Widgeon-grass Ruppia maritima is common, growing in brackish water or low-salinity pools in salt marshes. Seagrass meadows are now much reduced, probably due to elevated nitrogen and increased sedimentation. In sounds, numerically dominant benthic taxa include bivalves, polychaetes and amphipods, many showing gradients in community type from mesohaline areas of the eastern shore to near marine salinities in western parts. The semi-enclosed sounds have extensive shellfisheries, especially of blue crab, northern quahogs, eastern oysters, and shrimp. Problems include contamination of some sediments with toxic substances, especially of metals and PCBs at sufficiently high levels to depress growth of some benthic macroinvertebrates. Numerous fish kills have been caused by toxic Pfiesteria outbreaks, and fish kills and habitat loss have been caused by episodic hypoxia and anoxia in rivers and estuaries. Oyster beds currently are in decline because of overharvesting, high siltation and suspended particulate loads, disease, hypoxia, and coastal development. Fisheries monitoring which began in the late 1970s shows greatest recorded landings in 1978–1982; since then, harvests have declined by about a half. Some management plans have been developed toward improving water quality and fisheries sustainability. Major challenges include; high coliform levels leading to closures of shellfish beds, a problem that has increased with urban development and increasing cover of watershed by impervious surfaces; high by-catch and heavy trawling activity; overfishing which has led to serious declines in many wild fish stocks; and eutrophication. Comprehensive plans limiting nutrient inputs are needed for all coastal rivers and estuaries, not only those that already exhibit problems. There is a critical need to improve management of non-point nutrient runoff through increased use of streamside vegetated buffers, preservation of remaining natural wetlands and construction of artificial wetlands. Improved treatment processes, based on strong incentive programmes, should also be mandated for present and future industrial-scale animal operations.

Using transplanted oyster (Crassostrea virginica) beds to improve water quality in small tidal creeks: a pilot study

Abstract The Eastern oyster, Crassostrea virginica , may improve water quality by filtering large quantities of particulate matter (both organic and inorganic) and nutrients from the overlying water column. Additionally, oyster reefs alter hydrodynamic conditions, further increasing the removal of particulate matter from the water column. This study examined the effects of small-scale oyster additions on sediment loading, chlorophyll a , nutrient concentrations, and flow in small tidal creeks. Two reefs were established in Hewletts Creek, New Hanover County, North Carolina. Total suspended solids (TSS), chlorophyll a , and ammonium were measured upstream and downstream of each created reef and in an adjacent control channel that lacked a reef. Data were collected monthly during ebb tides over a 10-month period between September 2000 and June 2001. In the first month after initial reef placement, mean TSS concentrations downstream of reef placement were slightly lower than those upstream of the reef. Although not statistically significant, TSS concentrations downstream of the reefs were less than upstream concentrations for five out of nine and five out of seven post-reef sampling months for the upland and the lower creek sites, respectively. Chlorophyll a concentrations were not significantly affected by initial reef placement (2×3 m), but were reduced substantially after reef enlargement (3×4 m) in one of the experimental creeks. Reef placement resulted in significant increases in ammonium concentrations downstream of the transplanted-reefs. In addition, deposition of feces and pseudofeces by the oysters resulted in accumulation of finer-grained materials in the treated channel relative to the control channels. Oyster filtration was most effective three hours following high tide, when the ratio of flow discharge to reef surface area was the highest. This work demonstrates that small oyster reefs established and maintained in some small tributary channels can reduce TSS and chlorophyll a concentrations and that the magnitude of the effect may vary over the course of the tidal cycle.

Top down vs. bottom up control of benthic community composition on an intertidal tideflat

Nutrient enrichment and predation have been shown to have important, though sometimes conflicting, effects on intermediate trophic levels in freshwater communities. Though predation is also important in structuring marine soft-sediment benthic communities, the influence of nutrient enrichment in marine benthic systems is less well understood. We examined the interactive effects of nutrient enrichment and predator exclusion on a benthic community to assess the importance of these factors and to test the bottom-up:top-down theory of trophic control in this habitat. Predator exclusion was associated with increased numbers of surface deposit feeders but had less impact on burrowing deposit feeders. Effects of nutrient addition on abundances were observed only in treatments allowing predator access, but increased size of a benthic polychaete, Streblospio benedicti Webster, was observed in predator exclusion treatments with nutrient addition relative to other exclusion plots. However, responses to nutrient addition and predator exclusion varied between years. These results suggest that nutrient addition may be associated with effects on benthic communities, possibly through increased benthic microalgal production, but that the presence or absence of predation may alter the visible response of benthos to enrichment.

Top-down Versus Bottom-up Limitation in Benthic Infaunal Communities: Direct and Indirect Effects

Top-down effects of predators and bottom-up effects related to resource availability can be important in determining community structure and function through both direct and indirect processes. Their relative influence may vary among habitats. We examined the effects of nutrient enhancement and predation in southeastern North Carolina to determine relative effects on benthic macrofaunal communities. Short-term nutrient additions and predator exclusions were conducted in two estuaries to examine main and interactive effects on benthic microalgae and infauna. This experimental approach was complemented by comparisons of microalgal biomass, infaunal abundance and composition, predator abundance and predator exclusion among four estuarine systems that varied in background nutrient levels. In the short-term experiments, nutrient enhancement induced increased microalgal biomass but had limited effects on abundances or sizes of infauna. Predator exclusion increased the density of sedentary and near-surface dwelling fauna, but we did not observe interactions between predation and responses to nutrient additions as might be predicted from a simple cascade model. General patterns of abundance were explained to a larger extent by interannual and amongestuary pattems. These results indicate a lack of simple trophic cascade responses for this community over a short time scale and little evidence for local interactive effects. The lack of interactive effects may reflect the opportunistic nature of the dominant infaunal species and potentially different time and spatial scales for the effects of predation and resource controls.

Plant and infaunal communities associated with a created marsh

Destruction of tidal wetlands has led to a growing interest in the restoration and creation of new wetland habitat. However, while natural stands of vegetation have been successfully duplicated, less is understood about the establishment of faunal communities in created or restored tidal marshes. Infauna, which may form an important link between detrital production and commercially important finfish and decapods, have received limited attention in vegetated marsh habitats. We examined the infauna, changes in vegetation composition, and selected physical parameters in created marshes of different ages. Infauna were sampled using standard core sampling techniques. Vegetation composition and changes in relative abundance were observed using plot-point techniques. Vegetation plots indicated ongoing replacement ofSpartina alterniflora bySchoenoplectus robustus, a pattern supported by comparisons of vegetation at one of the sites to that reported in a previous study. Infauna exhibited significant differences between sites of different ages, with the intermediate-age site having intermediate densities for several taxa. These results suggest that both infauna and vegetation in created marshes undergo long-term change (ongoing after 10–20 yr), with both the plant and infaunal communities having qualitatively similar overall species composition to natural marsh areas.