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Evidence for the Physical Effects of Catchment Sediment Runoff

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Evidence for the Physical Effects of Catchment Sediment Runoff Evidence for the physical effects of catchment sediment runoff preserved in estuarine sediments: Phase III (macrofaunal communities) June 2003 Technical Publication 222 Auckland Regional Council Technical Publication No. 222, June 2003 ISBN 1-877353-26-4, ISSN 1175 205X www.arc.govt.nz on recycled paper Evidence for the physical effects of catchment sediment runoff preserved in estuarine sediments: Phase III macrofaunal communities C.J. Lundquist K. Vopel S.F. Thrush A. Swales Prepared for Auckland Regional Council NIWA Client Report: HAM2003-051 June 2003 NIWA Project: ARC03202 National Institute of Water & Atmospheric Research Ltd Gate 10, Silverdale Road, Hamilton P O Box 11115, Hamilton, New Zealand Phone +64-7-856 7026, Fax +64-7-856 0151 www.niwa.co.nz Contents Executive Summary iv 1. Introduction 1 1.1 Background 1 1.2 Study objectives 2 2. Methods 3 2.1 Sampling design 3 2.2 Sampling locations 3 2.3 Sampling design and sample processing 6 2.4 Macrofaunal analyses 7 2.5 Statistical analyses 7 2.6 Linking community analyses to environmental variables 8 3. Results 9 3.1 Description of communities found within each Auckland estuary 9 3.1.1 Mahurangi estuary (intertidal and subtidal cores) 9 3.1.2 Puhoi estuary (intertidal cores) 16 3.1.3 Okura estuary (intertidal cores) 17 3.1.4 Waitemata estuary (intertidal and subtidal cores) 19 3.1.5 Whitford embayment (subtidal cores) 22 3.1.6 Wairoa estuary (intertidal and subtidal cores) 23 3.1.7 Te Matuku estuary (Waiheke Island) (intertidal cores) 25 3.2 Comparison of communities within Auckland estuaries 27 3.2.1 Subtidal and intertidal comparisons 27 3.2.2 Across estuary comparisons 29 3.3 Predicting community composition with environmental data 33 4. Conceptual Model 37 5. Conclusions and Recommendations 39 6. Acknowledgements 43 7. References 45 __________________________________________________________________________________ Executive Summary Macrofaunal community structure is determined by a number of factors that can be influenced by sedimentation rates. Sedimentation in estuaries modifies estuarine morphology and sediment type, increases levels of suspended sediments, and changes the food resources available to macrofaunal communities. Sediment disturbances can be both chronic and catastrophic; however, the frequency of large events is likely to be particularly important in determining the abundance and structure of benthic communities. Stratigraphic cores of estuarine sediments can be analysed to determine rates of sedimentation, changes in sediment grain size, and mass mortality events of bivalves preserved as layers of shells. These time series of sedimentation can be compared with present-day macrofaunal community data to allow us to interpret the long-term ecological effects of sedimentation history on macrofaunal communities. This report is the third and final report of a three part series. In the first report, we reviewed what was known about the effects of catchment sediment runoff in estuaries and suggested procedures and methods to obtain estimates of sedimentation rates. In the second report, we presented analyses of sediment cores collected from Auckland estuaries (drowned valley estuaries: Mahurangi, Okura, Waitemata, Henderson, and Wairoa; tidal lagoons: Puhoi; and coastal embayments: Whitford, Te Matuku – Waiheke Island). In these analyses, pollen, caesium-137 (137Cs), lead-210 (210Pb), Zinc (Zn) and sediment particle size were used to determine sediment accumulation rates (SAR) for each of the estuaries. In this third report, we present information on the communities of benthic macrofauna found in the areas from which the sediment cores were taken, as a regional ‘snap-shot’ of macrobenthic biodiversity in Auckland estuaries. Although muddy areas are generally thought of as being of low diversity, we found this was not necessarily the case. Instead, we found a range of community types in Auckland estuaries, varying among estuarine type as well as between subtidal and intertidal locations within individual estuaries. Two estuaries had notably low diversity, Puhoi Intertidal and Wairoa Intertidal estuaries, corresponding to the highest rates of sediment accumulation calculated in the Phase II study. We present a conceptual model that can be used to predict likely changes in macrofaunal communities as Sediment Accumulation Rate (SAR) increases in estuaries, as may happen with increasing urbanisation. The model links community structure with aging of the estuary, incorporating predictable changes in macrofaunal community structure as estuaries progress from relatively open basins with large proportions of subtidal habitat to largely infilled areas of primarily intertidal habitat. TP 222 - Evidence for the physical effects of catchment sediment runoff preserved in estuarine sediments: Phase III macrofaunal communities iv Both the conceptual model and macrobenthic community data indicate that all three types of estuaries studied (tidal lagoons, drowned valley estuaries and coastal embayments) are comprised of diverse, variable communities at early stages of estuarine infilling (corresponding to low or natural rates of sediment accumulation). The three types of estuaries converge to a similar low diversity, stress-tolerant community as SAR increases and estuaries become infilled. This research points to the need for further research on the long-term effects of sedimentation on different estuary types, as in this study, estuary types were not able to be examined at all stages of sedimentation. In addition, the study was limited to muddier sediments. As macrofaunal communities in sandier sediments are likely less tolerant of increased muddiness and higher levels of suspended sediment concentrations than are those in muddier sediments, studies of this type should be expanded to determine the effects of chronic and catastrophic levels of sediment deposition on these sandy communities. Finally, fundamental research is necessary to address the collection of physical data on sedimentation at scales relevant to the macrofaunal communities, in order to better analyse the effects of both net and gross sedimentation. TP 222 - Evidence for the physical effects of catchment sediment runoff preserved in estuarine sediments: Phase III macrofaunal communities v 1. Introduction 1.1 Background The Auckland region is currently home to about a million people and a doubling of the population is anticipated to occur during the next 50 years. Urban development increases runoff of water and sediment during the earthworks phase, and contaminant loads increase as urban areas mature. Both of these factors can alter the composition and quantity of sediment deposited in estuaries (Williams, 1976; Douglas, 1985; Williamson, 1993; Morrissey et al. 2000; Swales et al. 2002). While some of this catchment sediment runoff is discharged directly to the sea, part of the sediment load is deposited in estuaries. Estuaries are particularly vulnerable to the effects of catchment land-use changes because of their close proximity to the sediment source and physical and biological processes that promote sedimentation. Auckland’s numerous and varied estuaries are highly valued features of the coastal environment, and are variously impacted by land runoff. Evidence from prior studies (reviewed in Hume et al. 2002) has linked catchment development and sediment accumulation to ecological effects on estuarine macrofaunal communities. Long-term monitoring programmes have implicated increased quantities of fine sediments in the decline of populations of the horse mussel Atrina zelandica and changes in the structure of benthic faunal communities (Mahurangi estuary: Cummings et al. 2003. More direct and stronger evidence of the effects of mud deposition on benthic fauna is provided by biological processes studies that have directly examined the effects of sudden increases in the rate of sedimentation (Hewitt et al. 1998, Norkko et al. 1999, Nicholls et al. 2000, Lohrer et al. 2003) and increased levels of suspended sediments (Thrush et al. 1998, Ellis et al. 1999, Hewitt et al. 2001). In this final phase of a three-part study, we identified robust indicators that can be used to quantify the physical effects of catchment sediment runoff on estuaries. In Phase I of our study, Hume et al. (2002) reviewed the relative merits of methods commonly used to reconstruct impacts of catchment development on estuaries based on analysis of sediment cores. In Phase II, we used these indicators to determine rates of sediment accumulation in seven estuaries (subtidal and intertidal environments) within the Auckland region (Swales et al. 2002). TP 222 - Evidence for the physical effects of catchment sediment runoff preserved in estuarine sediments: Phase III macrofaunal communities 1 1.2 Study objectives The objectives of the third phase of this study were to investigate the structural similarities of macrofaunal communities at ten locations with the Auckland region. These are the same sites that were analysed with sediment stratigraphic cores in Phase II. We present a regional ‘snap-shot’ of macrobenthic biodiversity within this cross-section of estuaries in the Auckland region, relating historical land-use patterns to current estuarine structure and macrofaunal communities. Hume et al. (2002) argued that the effects of catchment sediment runoff on macrofaunal communities may not be captured in the stratigraphic record. Rather, stratigraphic records provide information about historical changes in net sedimentation rates, not
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