Environmental state of a small intertidal estuary a decade after mangrove clearance, Waikaraka Estuary, Aotearoa New Zealand

Abstract

Unlike many parts of the world, mangrove coverage has been steadily increasing in Aotearoa,New Zealand since the 1900s. Intentional mangrove removal in Aotearoa New Zealand is often motivated by a desire to remove muddy sediment from sites that were once sand-dominated. Mangrove removal can result in geomorphic and ecologic evolution over decadal timescales, yet monitoring of removal sites is often limited to a few years. This study presents the result of over a decade of monitoring in Waikaraka Estuary, a quiescent embayment where mangroves were removed between 2003 and 2012. Surface elevation was monitored between 2006 and 2019, and sediment cores and surface samples collected in 2019 were analysed for grain size, total organic content, and root biomass to assess geomorphic change. Remaining mangroves areas and benthic macrofauna were surveyed throughout the estuary to assess ecologic change. Despite initially rapid movement of mud out of cleared areas, the estuary has experienced little bed-elevation change over a decade. The upper estuary remains muddy, and much of the root biomass has not decomposed. However, the region near the mouth of the estuary has become sandier with the return of bivalve species Austrovenus stutchburyi (cockles) and Macomona liliana. These long-term measurements demonstrate the slow pace of recovery following mangrove removal in quiescent embayments. Without major changes to hydrodynamics, mud is unlikely to be flushed out and bivalve habitat is unlikely to be restored even after a decade. Site-specific characteristics should be assessed when evaluating the efficacy of mangrove removal for estuary restoration, to moderate community expectations.