1. Only 800 years ago, New Zealand became the last major land mass to be settled by humans, leading to environmental degradation and precipitating a decline in indigenous fauna. Such ecological downgrading can alter ecosystem processes and drive down the capacity for remnant ecosystems to withstand the anthropogenic pressures of today.2. In New Zealand, these impacts are chronicled in a concise and remarkably complete archaeological record and are distinguishable from natural changes due to changing climate.3. Estuaries are subject to strong environmental gradients that drive productivity and contain critical habitats for key life-history phases for marine and terrestrial species. Linking land to sea, they host abundant, accessible human resources and are sensitive to anthropogenic environmental modification. 4. Austrovenus stutchburyi is a common filter feeding bivalve in New Zealand estuar-ies. An important food source for Māori, their shells are abundant in middens.Growth rates of A. stutchburyi are affected by multiple environmental factors, including temperature, salinity, nutrients, and sediment, and are recorded through time as easily measurable annual shell bands. Measuring spatial and temporal variation in growth rate of A. stutchburyi can be used to identify the timing of changes in estuarine conditions. Growth bands were measured in archaeological and modern (AD 1300-present)A. stutchburyi shells from six sites around New Zealand with varying occupational histories. There were no increases in growth rate, and rates declined measurably over time at three sites (up to a 50% reduction in growth per year). This decline was greater at estuaries that had experienced greater catchment modification, indicating that sediment loading due to land clearance was a likely driver of this change.6. New Zealand's coastline and marine resources are often perceived as wild and pristine. We propose that fundamental changes have occurred in the functioning of coastal marine ecosystems, which constrain its future.