S. J. Turner

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.

Spatial structure of bivalves in a sandflat:: Scale and generating processes

A survey was conducted during the summer of 1994 within a fairly homogeneous 12.5 ha area of sandflat off Wiroa Island, in Manukau Harbour, New Zealand, to identify factors controlling the spatial distributions of the two dominant bivalves, Macomona liliana Iredale and Austrovenus stutchburyi (Gray), and to look for evidence of adult–juvenile interactions within and between species. Most of the large–scale spatial structure detected in the bivalve count variables (two species, several size classes of each) was explained by the physical and biological variables. The results of principal component analysis and spatial regression modelling suggest that different factors are controlling the spatial distributions of adults and juveniles. Larger size classes of both species displayed significant spatial structure, with physical variables explaining some but not all of this variation. Smaller organisms were less strongly spatially structured, with virtually all of the structure explained by physical variables. The physical variables important in the regression models differed among size classes of a species and between species. Extreme size classes (largest and smallest) were best explained by the models; physical variables explained from 10% to about 70% of the variation across the study site. Significant residual spatial variability was detected in the larger bivalves at the scale of the study site. The unexplained variability (20 to 90%) found in the models is likely to correspond to phenomena operating at smaller scales. Finally, we found no support for adult–juvenile interactions at the scale of our study site, given our sampling scale, after controlling for the effects of the available physical variables. This is in contrast to significant adult–juvenile interactions found in smaller–scale surveys and in field experiments. Our perception of adult–juvenile interactions thus depends on the scale of study.

Bedload and water-column transport and colonization processes by post-settlement benthic macrofauna: Does infaunal density matter?

Copyright (c) 1997 Elsevier Science B.V. All rights reserved. In this study, we document dispersal and colonization by post-settlement benthic macrofauna on a large (250×500 m) area of intertidal sandflat at Wiroa Island in the Manukau Harbour, New Zealand, over a 2-week period in February 1994. We examine the effects of variation in the natural density of large (>15 mm) Macomona liliana Iredale (a tellinid bivalve) on these processes. Post-settlement transport was measured using cylindrical bedload and water-column traps positioned at 22 experimental sites within the study site. Macrofaunal colonization was documented using small pans of defaunated sediment deployed at each of the experimental sites. Greatest macrofaunal colonization of pans occurred in areas of greatest sediment reworking and deposition, suggesting transport and deposition of post-settlement stages may be passive processes. For some species [e.g. Austrovenus stutchburyi (Wood)] there was a significant positive relationship between their abundance and the weight of sediment in bedload traps, as well as wind conditions over the study period. This is consistent with passive transport processes, but does not exclude the possibility of active dispersal processes also being important. Differences in the hydrodynamic conditions (e.g. increased wind generated wave activity) over the 2-week study may have contributed to changes in abundance on the 2 sampling dates. For other species (e.g. Macomona liliana) the absence of significant positive relationships between abundance in either bedload or water-column traps and the weight of sediment retained in traps, as well as between ambient densities and the net flux of individuals across sites, is consistent with active dispersal and colonization. There was a negative relationship between background density of large Macomona liliana and numbers of smaller conspecifics (≤4 mm) and other bivalves [Austrovenus stutchburyi, Cyclomactra ovata (Gray) and Nucula hartvigiana Pfeiffer] colonizing pans of defaunated sediment. The results suggest that 10 to 100s m scale variation in the natural densities of an abundant siphonal surface deposit feeder may have a significant influence on dispersal and colonization and, thus, the spatial patterns of macrofauna on the tidal flat.

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.

Scaling-up from experiments to complex ecological systems: Where to next?

a , b c d e * S.F. Thrush , D.C. Schneider , P. Legendre , R.B. Whitlatch , P.K. Dayton , a f a g h J.E. Hewitt , A.H. Hines , V.J. Cummings , S.M. Lawrie , J. Grant , a a i R.D. Pridmore , S.J. Turner , B.H. McArdle National Institute of Water and Atmospheric Research, P.O. Box 11-115, Hamilton, New Zealand Ocean Sciences Centre, Memorial University of Newfoundland, St. John’s, Canada ALC5S7 c ́ ́ ́ Departement de Sciences Biologiques, Universite de Montreal, C.P. 6128, succursale Centre-ville, ́ ́ Montreal, Quebec H3C 3J7, Canada Dept. Marine Sciences, University of Connecticut, Groton, CT 06340-6097, USA Scripps Institution of Oceanography, University of California San Diego, La Jolla, CA 92093-0201, USA Smithsonian Environmental Research Center, P.O. Box 28, Edgewater, MD 21037, USA Culterty Field Station, University of Aberdeen, Newburgh, AB40AA, Scotland Dept. of Oceanography, Dalhousie University, Halifax, Canada B3H 4JI Biostatistics Unit, School of Biological Sciences, University of Auckland, Private Bag 92019, Auckland, New Zealand

Matching the outcome of small-scale density manipulation experiments with larger scale patterns: an example of bivalve adult/juvenile interactions

Generalising or scaling up from small-scale experiments to larger areas is an important challenge for both ecology and conservation biology. This study describes a technique that attempts to meet this challenge by combining spatial mapping with small-scale process experiments. Specifically, we evaluate the density effects of large individuals (>15 mm shell length) of a tellinid bivalve (Macomona liliana Iredale) on macrofauna in 0.25 m2 experimental plots within the natural density variation of large Macomona over a 12.5 ha site. By mapping the spatial distribution of large Macomona before conducting the experiment, we were able to identify homogeneous areas with different background densities of large Macomona and embed 22 experimental locations within the natural density-scape. Within each location, four experimental densities were added to plots from which all large macrofauna (>4 mm) had been previously removed. Macrofauna were sampled 22 days after the start of the experiment and significant negative treatment effects of high densities of large Macomona were identified by ANOVA for juvenile bivalves Macomona (<4 mm), Austrovenus stutchburyi (Gray) (<4 mm), the isopod Exosphaeroma falcatum Tattersall and the total number of individuals. Generalised linear models were then used to include the effect of background density variation of large Macomona in the analysis. Only Austrovenus (<4 mm) demonstrated a significant interaction between the background and experimental densities of large Macomona. This resulted from background densities of large Macomona having a significant effect on Austrovenus (<4 mm) in the two lowest density treatments only. Significant effects were detected only because we had planned the study to cover the various background densities of Macomona. The effect of experimental and background density variation of large Macomona on Macomona (<4 mm), Exospheroma, nemerteans and the total number of individuals were similar in direction and strength. Except for nemerteans, all relationships were negative, with low densities of macrofauna associated with high experimental and background densities of large Macomona. This implies that large-scale extrinsic factors (e.g., elevation, exposure to wave disturbance) are not the only features influencing the distribution of Macomona at the scale of the study site; intrinsic processes operating on smaller scales are also important. This scale-dependent response would not have been uncovered, had we not conducted a larger-scale survey in concert with the smaller-scale experiment.

Factors affecting the distribution of benthic macrofauna in estuaries contaminated by urban runoff

Contaminants derived from urban runoff have been shown to accumulate in estuarine sediments, reaching concentrations potentially capable of causing biological effects. Demonstration of effects, however, is difficult due to strong natural environmental gradients and the effects of past or present point-sources of contamination. We used multivariate methods to test two hypotheses relating to the effects of urban-derived contaminants on estuarine benthic communities. First, that patterns of distribution and abundance of benthic invertebrates in two urbanised estuaries would be different from those in two non-urbanised estuaries. Second, that the distributions of benthic invertebrates within and among the four estuaries would be related to those of urban-derived contaminants. Concentrations of contaminants were larger in estuaries with urbanised catchments and concentrations of Cu, Pb, Zn and DDT in some samples exceeded those at which biological effects may be expected to appear. Tests of differences in composition of benthic communities among estuaries showed that the two urban estuaries were not significantly different, but that they differed from both rural estuaries, which also differed from each other. Distributions of benthic invertebrates were significantly related to those of environmental variables, and were ordinated along axes that correlated with both natural environmental variables (nature of the sediment, position in estuary) and contaminants. Differences in faunas between the urban and non-urban estuaries were not, however, clear-cut and nor were relationships between faunal assemblages and environmental variables (including contaminants) consistent between two times of sampling.

The sandflat habitat: scaling from experiments to conclusions

a , a a a b * S.F. Thrush , R.D. Pridmore , R.G. Bell , V.J. Cummings , P.K. Dayton , c d a a e a R. Ford , J. Grant , M.O. Green , J.E. Hewitt , A.H. Hines , T.M. Hume , f g h a i S.M. Lawrie , P. Legendre , B.H. McArdle , D. Morrisey , D.C. Schneider , a j k a S.J. Turner , R.A. Walters , R.B. Whitlatch , M.R. Wilkinson National Institute of Water and Atmospheric Research, P.O. Box 11-115, Hamilton, New Zealand Scripps Institution of Oceanography, University of California San Diego, La Jolla, CA 92093-0201, USA Dept. Marine Science, University of Otago, P.O. Box 56, Dunedin, New Zealand Dept. of Oceanography, Dalhousie University, Halifax, Canada B3H 4JI Smithsonian Environmental Research Centre, P.O. Box 28, Edgewater, MD 21037, USA Culterty Field Station, University of Aberdeen, Newburgh, AB40AA, Scotland g ́ ́ ́ Departement de Sciences Biologiques, Universite de Montreal, C.P. 6128, succursale Centre-ville, ́ ́ Montreal, Quebec H3C 3J7, Canada Biostatistics Unit, School of Biological Sciences, University of Auckland, Private Bag 92019, Auckland, New Zealand Ocean Sciences Centre, Memorial University of Newfoundland, St John’s, Canada ALC5S7 United States Geological Survey, 1201 Pacific Ave, Suite 600, Tacoma, WA 98402, USA Dept Marine Sciences, University of Connecticut, Avery Point, Groton, CT 06340-6097, USA

Patterns of sediment reworking and transport over small spatial scales on an intertidal sandflat, Manukau Harbour, New Zealand

Copyright (c) 1997 Elsevier Science B.V. All rights reserved. Measurements of physical sediment reworking and transport were conducted at 22 experimental sites within a 250×500 m study site on a sandflat at Wiroa Island (Manukau Harbour, New Zealand), in order to examine spatial patterns of sediment transport, and its relationship to passive advection of benthic fauna (Turner et al., 1997). Sediment reworking and transport were measured four times during February 1994 as replacement of dyed sand in pans of sediment buried in the intertidal zone, change in total height of the sediment column in the pans, and as deposition in tube traps with openings flush with the bed (bedload traps) and at 15 cm above the bed (water-column traps). Sediment reworking replaced about 2-3 mm of sand per day, with increasing cumulative transport to a depth of 20 mm during the study period. In addition, there were site-specific differences among sampling dates. Spatial structure in sediment reworking was analyzed by trend surface analysis. Depending on date, variance in reworking was influenced by location within the study site, tidal shear stress (model generated), and elevation on the sandflat. Analysis of residuals demonstrated that sediment reworking at times contained inherent spatial structure after accounting for the effects of other explanatory variables. Bedload trap rates in the final sampling period accounted for most of the variance in deposition indicated by sediment height. Sediment reworking and transport are variable over scales of 10 1 -10 2 m, as well as over a period of days, such that measurements determined in single point studies cannot necessarily be extrapolated over larger spatial scales. Patterns of sediment reworking and transport patterns provide a template against which to compare patterns of faunal transport. However, the linkage will be most apparent when 1) sediment reworking and transport are substantial in magnitude, 2) there is significant XY spatial structure to the pattern of sediment transport at the scale of the study, and 3) the fauna of interest are at least potentially transported as bedload (e.g. shelled forms).

CHANGES IN EPIFAUNAL ASSEMBLAGES IN RESPONSE TO MARINA OPERATIONS AND BOATING ACTIVITIES

Abstract A number of features of epifaunal assemblages suggest that they have the potential to provide an excellent integrated field-based measure of marine environmental quality. In this study we have used epifaunal assemblages to investigate the ecological effects of potential gradients of environmental stress arising from marina operations and boating activities. Replicate slate panels, initially colonized by solitary ascidian-dominated assemblages at a site remote from marina operations, were placed at five sites in or near each of two boat mooring areas, Okahu Bay boat harbour and Pine Harbour marina, in the Waitemata Harbour, New Zealand. In addition, panels were immersed at Station Bay, a site remote from marina operations and boating activities. Sites were positioned along putative gradients of contaminant levels (e.g. copper and zinc), as well as hydrodynamic and sedimentation regimes. The changes in assemblage structure on panels immersed at sites along these gradients were compared. After 3–6 months, significant differences were evident among the epifaunal assemblages along the gradients of environmental stress. The most conspicuous effect was the loss of cover by the abundant and spatially dominant solitary ascidians at sites inside the marinas. Concomitant with this loss, was a marked increase in space availability, and a small increase in the cover of sponges, hydroids, erect and encrusting bryozoans, and colonial ascidians. There were also significant differences in assemblage structure between Pine Harbour and Okahu Bay, as well as between the marinas and Station Bay.