Greig A. Funnell

DISTURBANCE OF THE MARINE BENTHIC HABITAT BY COMMERCIAL FISHING: IMPACTS AT THE SCALE OF THE FISHERY

Commercial fishing is one of the most important human impacts on the marine benthic environment. One such impact is through disturbance to benthic habitats as fishing gear (trawls and dredges) are dragged across the seafloor. While the direct effects of such an impact on benthic communities appear obvious, the magnitude of the effects has been very difficult to evaluate. Experimental fishing-disturbance studies have dem- onstrated changes in small areas; however, the broader scale implications attributing these changes to fishing impacts are based on long-term data and have been considered equivocal. By testing a series of a priori predictions derived from the literature (mainly results of small-scale experiments), we attempted to identify changes in benthic communities at the regional scale that could be attributed to commercial fishing. Samples along a putative gradient of fishing pressure were collected from 18 sites in the Hauraki Gulf, New Zealand. These sites varied in water depth from -17 to 35 m and in sediment characteristics from -1 to 48% mud and from 3 to 8.5 (Lg chlorophyll a/cm3. Video transects were used for counting large epifauna and grab/suction dredge and core sampling were used for collecting macrofauna. After accounting for the effects of location and sediment characteristics, 15-20% of the variability in the macrofauna community com- position sampled in the cores and grab/suction dredge samples was attributed to fishing. With decreasing fishing pressure we observed increases in the density of echinoderms, long- lived surface dwellers, total number of species and individuals, and the Shannon-Weiner diversity index. In addition, there were decreases in the density of deposit feeders, small opportunists, and the ratio of small to large individuals of the infaunal heart urchin, Echino- cardium australe. The effects of fishing on the larger macrofauna collected from the grab/ suction dredge samples were not as clear. However, changes in the predicted direction in epifaunal density and the total number of individuals were demonstrated. As predicted, decreased fishing pressure significantly increased the density of large epifauna observed in video transects. Our data provide evidence of broad-scale changes in benthic communities that can be directly related to fishing. As these changes were identifiable over broad spatial scales they are likely to have important ramifications for ecosystem management and the development of sustainable fisheries.

Seagrass patches and landscapes: The influence of wind-wave dynamics and hierarchical arrangements of spatial structure on macrofaunal seagrass communities

The spatial arrangement of seagrass beds varies from scales of centimeters to meters (rhizomes, shoot groups), meters to tens of meters (patches), to tens of meters to kilometers (seagrass landscapes). In this study we examine the role of patch scale (patch size, seagrass % cover, seagrass biomass), landscape scale (fractal geometry, patch isolation) and wave exposure (mean wind velocity and exceedance) variables in influencing benthic community composition in seagrass beds at three intertidal sites in northern New Zealand (two sites in Manukau Harbour and one site in Whangapoua Harbour). Analysis of univariate community measures (numbers of individuals and species, species richness, diversity and evenness) and multivariate analyses indicated that there were significant differences in community composition inside and outside of seagrass patches at each of the three sites. Partialling out the spatial and temporal components of the ecological variation indicated that seagrass patch variables explained only 3–4% of the patch scale variation in benthic community composition at each of the sites. The temporal component was more important, explaining 12–14% of the variation. The unexplained variation was high (about 75%) at all three sites, indicating that other factors were influencing variation in community composition at the scale of the patches, or that there was a large amount of stochastic variation. Landscape and wave exposure variables explained 62.5% of the variation in the species abundance data, and the unexplained variation at the landscape level was correspondingly low (12%). Canonical correspondence analysis produced an ordination that suggests that, while mean wind velocity and exceedance were important in explaining the differences between the communities in the two harbours, spatial patterning of the habitat, primarily fractal dimension, and secondarily patch isolation (or some factors that were similarly correlated), were important in contributing to variability in community composition at the two sites in Manukau Harbour. This study suggests that spatial patterning of seagrass habitat at landscape scales, independent of the patch scale characteristics of the seagrass beds, can affect benthic community composition. Community composition inside and outside seagrass habitats involves responses to seagrass bed structure at a series of hierarchical levels, and we need to consider more than one spatial scale if we are to understand community dynamics in seagrass habitats.

THE GENERALITY OF FIELD EXPERIMENTS: INTERACTIONS BETWEEN LOCAL AND BROAD‐SCALE PROCESSES

Linking the results of localized field experiments to generalities about the role of specific processes is essential in ecology. Comparative studies conducted at multiple locations enable the general importance of processes to be assessed. However, spatial or temporal variation in the strength of local ecological relationships frequently makes it difficult to draw general conclusions, as increasing the extent of a study is likely to increase the physical and biological heterogeneity. To unravel the influence of differences in wave climate on local ecological interactions among adult and juvenile sandflat bivalves, an experiment was conducted at three sites in each of three harbors in the North Island of New Zealand. The selected sites covered a range of wind-wave exposures but were all mid-intertidal sandflats with macrobenthic communities dominated by bivalves. Four treat- ments were used: additions of 0, 15, 45, and 110 adults of the tellinid bivalve Macomona liliana. At each site a DOBIE wave gauge was used to provide time-series data on wave orbital speed at the seabed over the 35-d duration of the experiment. Significant experiment x location interactions indicated site-dependent variation in the strength and direction of treatment effects. However, multiple regression models based on site environmental char- acteristics were very successful in explaining differences between sites in the strength of experimental treatment effects (66-99% of the variance explained). We used the cube of the orbital wave velocity at the seabed (U3) as an index of energy dissipation by waves. Both the site average and maximum U3 were important variables explaining location- dependent treatment effects. Average U} increased the strength of the negative effects of experimental additions of adult Macomona on juvenile bivalves, presumably by increasing the opportunity for juvenile bivalves to be resuspended by small waves and transported away from areas with high adult densities. Maximum U3 decreased the strength of the experimental effects, probably by increasing the purely passive transport of juveniles with sediment bedload and thus obliterating patterns in the distribution of juvenile bivalves relative to adult Macomona. Hence, different aspects of the wave climate influenced adult- juvenile interactions in different ways. Ambient density of adult Macomona around the experimental sites was also an important factor. This multisite experiment confirmed the importance of local adult-juvenile interactions and suggested ways in which these inter- actions are influenced by local environmental characteristics. Where the influence of such broad-scale variables can be identified, linking broad-scale information to small-scale stud- ies that identify mechanisms can increase the generality of ecological experiments.

CONDITIONAL OUTCOMES OF FACILITATION BY A HABITAT-MODIFYING SUBTIDAL BIVALVE

Facilitation by habitat modifiers is common in ecological communities, but the potential for temporal and spatial variations in environmental conditions to modify the outcome of these interactions and influence the strength of feedbacks is poorly understood. Suspension-feeding bivalves are important habitat modifiers that can facilitate surrounding communities by providing refuge from predation and changing boundary flows and through the production of organically enriched biodeposits. However, numerous studies have highlighted the problem of finding generalizable patterns. We tested the strength and generality of the relationship between the large suspension-feeding bivalve Atrina zelandica and surrounding macrofauna and hypothesized that facilitation by Atrina is conditional and modulated by site-specific suspended sediment concentration (SSC), which influences the quantity and quality of biodeposit production. We found temporally consistent patterns of higher rates of biodeposition and increased abundance and species richness in close proximity to Atrina under low SSC conditions. Facilitation strength decreased with increasing SSC, suggesting that the facilitation effect of Atrina is reduced and reversed along this environmental stress gradient.

MAPPING OF MARINE SOFT‐SEDIMENT COMMUNITIES: INTEGRATED SAMPLING FOR ECOLOGICAL INTERPRETATION

Increasingly, knowledge of broad-scale distribution patterns of populations, communities, and habitats of the seafloor is needed for impact assessment, conservation, and studies of ecological patterns and processes. There are substantial problems in directly transferring remote sensing approaches from terrestrial systems to the subtidal marine environment because of differences in sampling technologies and interpretation. At present, seafloor remote assessments tend to produce habitats predominantly based on sediment type and textural characteristics, with benthic communities often showing a high level of var- iability relative to these habitat types. Yet an integration of information on both the physical features of the seafloor and its ecology would be appropriate in many applications. In this study, data collected from a multi-resolution nested survey of side-scan, single-beam sonar and video are used to investigate a bottom-up approach for integrating acoustic data with quantitative assessments of subtidal soft-sediment epibenthic communities. This approach successfully identified aspects of the acoustic data, together with environmental variables, that represented habitats with distinctly different epibenthic communities. The approach can be used, regardless of differences in data resolution, to determine location- and device- specific relationships with the benthos. When such relationships can be successfully de- termined, marine ecologists have a tool for extrapolating from the more traditional small- scale sampling to the scales more appropriate for broad-scale impact assessment, manage- ment, and conservation.

MACROBENTHIC RECOVERY PROCESSES FOLLOWING CATASTROPHIC SEDIMENTATION ON ESTUARINE SANDFLATS

Land use can exacerbate the rate of sediment delivery to estuaries. In par- ticular, for catchments with steep terrain and heavy, sporadic rainfall, changes in land use can increase the risk of catastrophic deposition of terrestrial sediment. One of the key issues in assessing the ecological significance of catastrophic sedimentation events is determining the rate of recovery of the macrobenthic community and understanding how physical and biological processes influence the recovery rate in different locations. We conducted a field experiment over 212 days to assess the impact of terrestrial sediment deposits at six sites on the intertidal sandflats of Whitianga Harbour (New Zealand). Differences in the sedi- mentary habitat as a result of the deposition of terrestrial sediment lasted for ;50 d, although these effects varied between different sediment properties and between sites. The deposition of terrestrial sediment had an immediate and negative effect on resident macrofauna, al- though complete defaunation of the experimental plots did not occur. Macrobenthic recovery lagged behind the recovery of the sediment properties. Based on multivariate analysis, three sites never recovered over the duration of the experiment, while based on univariate analysis four sites never recovered. Macrofaunal assemblages living deep (2-15 cm) within the sediment were generally slower to recover than those found in the top 2 cm. A meta-analysis of recovery rates for macrofauna was conducted on information generated in this study and in two similar experiments carried out in other New Zealand estuaries. This analysis revealed a consistent negative relationship between the magnitude of disturbance generated by the terrestrial sediment layer and the recovery of the macrobenthos. Measures of recovery at the community level emphasized the importance of site environmental factors reflecting increased wave disturbance, flow velocity, and the wetting and drying of the deposited terrestrial sediment in speeding recovery. Overall, our results indicate that the long-term effects of catastrophic sediment disturbance are influenced by local hydrodynamic condi- tions and the composition of resident macrofauna living in sediments adjacent to disturbed areas. Given the long time scale of recovery from these experiments they indicate the potential for catastrophic sediment deposition to result in broad-scale degradation of es- tuarine macrobenthic communities.

Accounting for local scale variability in benthos: implications for future assessments of latitudinal trends in the coastal Ross Sea

This paper is a contribution to the Latitudinal Gradient Project. It describes macro and epifaunal assemblages and habitats at three shallow water locations at the southern end of the western Ross Sea coast, and investigates relationships between faunal composition and environmental characteristics. Many variables (e.g. substrate type, sediment composition, depth, latitude, longitude) contributed to explaining the differences in community composition between locations, with latitude (a likely surrogate for broader scale factors, e.g. ice cover) one of the most important. The percentage explained by environmental characteristics was strongly scale dependent, decreasing with increasing scale of observation. As much as 66% and 75% of the variability in macrofaunal and epifaunal assemblages, respectively, was explained at the smallest scale (i.e. between transects within a location), compared to 9–18% and 11–32%, respectively, at the scale of the entire study. This relationship was also true for species richness and total abundance. This suggests that while small-scale habitat variability will not confound our ability to detect latitudinal gradients in future studies, adequately quantifying the environmental factors important in structuring these communities at larger (latitudinal) spatial scales will be important. Finally, large differences in habitat structure did not translate into large differences in the diversity of fauna, illustrating the difficulty of predicting faunal composition in the Ross Sea based on seafloor topography alone.

The role of waves in the colonisation of terrestrial sediments deposited in the marine environment

Elevated rates of sediment run-off, as a result of changes in land-use and climate, are a significant threat to marine coastal communities, with a potential to cause broad-scale, long-term alteration of habitats. Individual sedimentation events can smother estuarine flats with terrigenous sediments, creating a significant disturbance to local benthic communities. Variations in the degree to which a habitat is altered, the rate at which mixing occurs, and species-specific dispersal and responses to the altered habitat, suggest that colonisation of terrestrial sediment depositions will vary with location, both between and within estuaries. This study was designed to explore the effect that variations in wave-induced hydrodynamics would have on long-term colonisation of terrestrial sediment depositions on intertidal flats. Sites for the experimental deposition of terrestrial sediment were located along a gradient in wave exposure, with only limited variation in immersion times (30 min) and ambient sediment particle sizes (predominantly fine sand). Over 20 months, periodic measurements were made of factors predicted to affect colonisation: the sediment characteristics of the deposited sediment; local-scale wave climate; bioturbation of the deposited sediment; and local populations of benthic invertebrates. Neither opportunistic use of the new resource, progressive recovery or facilitation by colonising macrofauna was observed. Little vertical mixing of the deposited and existing sediment by either waves or bioturbators occurred; instead bedload transport was the dominant process. Local differences in hydrodynamic conditions and macrobenthic communities resulted in site-specific colonisation of the experimental plots. The strength and duration of the macrofaunal response to deposited sediment observed in this study suggest that chronic small-scale (ms) patchy deposition of terrestrial sediment in the intertidal marine environment has a strong potential to alter both habitats and communities.

Nearshore benthic community structure at the Bounty and Antipodes Islands, Subantarctic New Zealand

Management decisions aimed at protecting biodiversity ideally should be based on biological information, but for remote and logistically difficult sites, such as are found at high latitudes, these data may be lacking. During March 2009, surveys were completed of the nearshore rocky reef communities around the Bounty and Antipodes Islands, in New Zealand’s subantarctic region. Previously considered to support the same habitat types (which used physical variables as surrogates for biological communities), analysis of photoquadrats taken at both island groups showed that the rocky reef communities were significantly different, both in terms of their species composition and in terms of their potential ecological function. While Antipodes Island supported fairly typical subantarctic shallow subtidal marine communities dominated by nongeniculate coralline algae, the rocky reefs at the Bounty Islands were dominated by filter- and suspension-feeding invertebrates, in particular encrusting sponges, barnacles and mussels. The mobile invertebrate fauna associated with these communities were also significantly different between the two island groups. Contrasting geology, oceanographic conditions and nutrient input from seabird and pinniped colonies may all contribute to the observed nearshore community structures at the Bounty and Antipodes Islands. Our research provides a baseline for assessing change in the subantarctic region and highlights the importance of using biological community data where available, to inform conservation management decisions.