Reconstructing two decades of spatiotemporal change in estuarine ecological condition

Estuarine ecosystems deliver critical ecological functions and services yet face mounting cumulative pressures from nutrient enrichment, sedimentation, and contaminants. We applied a flexible spatial data framework to reconstruct spatiotemporal trajectories of change in ecological condition in New River Estuary (NRE - South Island, New Zealand) over five time periods (2000, 2002, 2008, 2012, 2020). Four ecological principle layers addressing ecosystem recovery potential, ecological network structure, resilience-related ecological processes, and seascape-scale habitat diversity were developed alongside a combined cumulative stressor layer integrating legacy, response, and stressor network-impact. Our findings showed significant declines in ecological network structure and resilience-related ecological processes through time, which suggests weakening network connectivity and resilience processes across NRE and indicating a decline in its ecological condition consistent with oral histories and observations of change in the now highly degraded estuary. We also found that the cumulative stressor values increased over time, with the strongest effects in the upper estuary, and significant contrasts between early (2000-2008) and late (2012-2020) periods. Furthermore, the observed spatiotemporal trends inferred from the flexible spatial data framework aligned with biogeochemical indicator records in dated sediment cores (total nitrogen, organic matter, δ¹⁵N, zinc, nickel, sediment accumulation rates), also suggesting an overall decline in estuarine ecological condition, validating the approach, and highlighting catchment-derived drivers of change. Our results demonstrate the utility of spatially continuous, cost-effective assessments to inform targeted restoration and adaptive management in data-limited environments.