Contamination on sandflats and the decoupling of linked ecological functions ANDREW M. LOHRER,* JUDI E. HEWITT, SARAH F. HAILES, SIMON F. THRUSH, MICHAEL AHRENS† AND JANE HALLIDAY‡ National Institute ofWater & Atmospheric Research (NIWA), PO Box 11-115, Hillcrest, Hamilton 3251, New Zealand (Email: d.lohrer@niwa.co.nz) Abstract Benthic macrofauna can influence inputs and transformations of energy and matter in estuaries, affecting both the stocks of vital materials (e.g. carbon, oxygen) and the rates of key processes (e.g. organic matter decomposition, nutrient uptake). Although a number of studies have identified shifts in functional groups or biological traits in relation to anthropogenic stressors, there have been few field-based assessments of changes in functioning associated with stress gradients.We used a comparative experimental approach to investigate function- ing on two sandflats with differing exposures to urban contaminants. Apart from significant differences in sediment contaminant concentrations (43.2  1.8 mg kg -1 Zn and 15.6  0.9 mg kg -1 Pb at the Pollen site; 17.7  0.7 mg kg -1 Zn and 7.9  0.9 mg kg -1 Pb at theWaiheke site), the two sandflats were readily comparable: both had similar sediment grain size distributions and were dominated by the same macrofaunal species; and both were in non-eutrophic New Zealand marine reserves with low ambient sediment organic matter content. To better understand the effects of contaminants on biologically mediated transformations of organic matter into inorganic nutrients, we manipulated sediment organic matter content and macrofaunal abundance in standardized treat- ments at each site. Fluxes of oxygen and ammonium, which are linked to key sandflat processes such as organic matter decomposition and benthic photosynthesis, were measured as response variables 1 week after the experi- mental manipulations.We predicted more efficient organic matter processing on the uncontaminated flat and thus expected to see elevated ammonium efflux in response to organic enrichment treatments at this site. Higher rates of benthic photosynthesis were predicted for plots with higher ammonium efflux, as ammonium is a readily utilizable form of limiting inorganic nitrogen.We documented significant positive relationships between ammonium uptake and benthic primary production on the uncontaminated flat, but weaker/insignificant relationships at the contaminated site. Our data were consistent with theories of increased variability and a decoupling of system processes with increasing amounts of stress. Key words: Austrovenus stutchburyi, ecosystem functioning, heavy metal, microphytobenthos, sandflat macrofauna. INTRODUCTION Estuaries are critical transition zones where inputs of energy and matter from terrestrial, freshwater and marine environments are processed and transformed (Levin et al. 2001). Living organisms modify the inputs and transformations of energy and matter in ecosystems, affecting both the stocks of vital materials (e.g. carbon, oxygen) and the rates of key processes (e.g. organic matter decomposition, nutrient uptake; Hooper et al. 2005). The organisms that live in estu- aries and affect essential estuarine functions have to cope with stress associated with strong physical gradi- ents and, increasingly, chronic pollution and multiple stressors (Dickinson et al. 1996; Adams 2005; Thrush et al. 2008; Koch et al. 2009). Losses of function in one part of an estuarine system (e.g. nutrient uptake by the benthos) due to a particular type of stressor (e.g. high contaminant concentrations in the sediment) can lead to broader changes in that estuary (e.g. harmful algal blooms, bottom water hypoxia, fish kills; Diaz & Rosenberg 1995; Cloern 2001; Kennish 2002) and cascading effects on functional performance. Thus, it is important to understand how estuarine organisms will respond to anthropogenic stressors and whether impacts on diversity are impairing essential ecosystem functions (Loreau et al. 2002; Hooper et al. 2005). Managing impacts to maintain multiple functions in *Corresponding author. †Present address: Facultad de Biologia Marina, Universidad de Bogota Jorge Tadeo Lozano, Carrera 4 No. 22-61, Bogota, Colombia.Tel: +57 1 242 7030, Email: michael.ahrens@utadeo. edu.co ‡Present address: National Institute of Water & Atmospheric Research (NIWA), Private Bag 14-901, Kilbirnie, Wellington, New Zealand. Tel: +64-4-386-0300, Email: j.halliday@niwa. co.nz Accepted for publication April 2010. Austral Ecology (2011) 36, 378–388 © 2010 The Authors doi:10.1111/j.1442-9993.2010.02148.x Journal compilation © 2010 Ecological Society of Australia