Penrhyn Estuary Habitat Enhancement Plan: Results of Monitoring Created Intertidal Benthic Habitats

Habitat enhancement at Penrhyn Estuary incorporated levelling of sand foredunes, creation and planting of intertidal saltmarsh habitat and extension and augmentation of intertidal sand flats to provide roosting habitats and... [ view full abstract ]

Habitat enhancement at Penrhyn Estuary incorporated levelling of sand foredunes, creation and planting of intertidal saltmarsh habitat and extension and augmentation of intertidal sand flats to provide roosting habitats and invertebrate prey for shorebirds. A key aim for this ambitious project was to increase quality and extent of roosting and feeding grounds for migratory shorebirds. The monitoring plan included surveys of benthic infaunal communities within the estuary and at reference locations within a BACI framework. This paper compares baseline data for estuarine sediments and invertebrate assemblages at Penrhyn Estuary and reference locations to conditions four years after completion of enhancement.
Criteria for the success of habitat enhancement were derived from a) comparison to target values derived from three pre-enhancement surveys in inner Penrhyn Estuary that were heavily utilised by shorebirds and b) comparison of rates of change at Penrhyn Estuary to those at reference locations. Physical indicators were median grain size and % composition of fine sediment (% clay and silt fractions) with target ranges of 0.31 – 0.34 mm and 2 – 4% respectively. Biological indicators were invertebrate abundance and biomass with target values of greater than 37 individuals per sample and 0.7 g/sample (or 89 g/m2) respectively. After enhancement the target for biomass in Penrhyn Estuary was exceeded, but was not significantly different at reference locations. Invertebrate abundance in Penrhyn Estuary reached only 61% of the target value and decreases were consistent with those in reference locations. Median grain size and percentage fines in created sand habitats were similar to pre-enhancement levels. Sediment characteristics, invertebrate abundance and biomass in sediments that received augmentation with seagrass wrack and river mud differed little from unaugmented areas 15 months after enhancement, indicating estuary-wide mixing of surface sediments.
Taxonomic composition of benthic assemblages shifted post enhancement. Polychaetes were characteristic of the assemblage before enhancement while gastropods and bivalve molluscs drove assemblage patterns after enhancement. Polychaetes declined from 76% of all invertebrates before enhancement to 47% after, while molluscs increased from 16% before to 49% after. Data also indicate smaller variation among taxonomic groups between sampling events after habitat enhancement compared to before.