The impact of green macroalgal mats on benthic invertebrates and overwintering wading birds.

Abstract

A consequence of increased nutrient levels within an estuarine ecosystem is the development of green macroalgal blooms or ‘mats’. These mats can cover extensive areas of estuarine intertidal habitats and have biomass >1 kg m-2 (wet weight). One of the key metrics for assessment of the ecological condition status of estuarine features in Europe is the extent and biomass of macroalgal mats. The aim of this research is to establish whether the development of green macroalgal mats affects feeding relationships between invertebrate assemblages and overwintering migratory wading birds in Poole Harbour - a temperate estuarine ecosystem on the south coast of England. Poole Harbour is designated for its populations of overwintering migratory wading birds. As such, any decline in wading bird numbers as a result of nutrient enrichment affecting their food supply or altering feeding behaviour, would result in sanctions under current legislation. This field research consisted of three main objectives: 1) Measuring the biomass and extent of the macroalgal mat within Poole Harbour. 2) Analysing any changes to the benthic invertebrate community under varying macroalgal mat densities. 3) Observing and recording the behaviour and feeding success of key wading bird species; in particular how they responded to changes in prey availability and varying levels of macroalgal mat coverage. Samples of macroalgal mat were taken monthly or bi-monthly on mudflats at four locations around the harbour over two years and wet weight biomass was recorded. Wading bird invertebrate prey availability was measured using benthic core samples taken at upper, mid, and lower shore levels at three key sites. Invertebrate size- classes were recorded and converted into available energy (kJ m-2) according to the preferred diet of each of the five wading bird species studied. Observations of wading bird behaviour were recorded over two overwintering periods (September – March). Digital video recordings were taken of different wading bird species’ feeding behaviour and success on varying levels of macroalgal mat coverage. Coverage by macroalgal mats was high (>50%) with dense patches persisting into autumn. Biomass of algae reached 1 kg m-2 at each of the sites during both years with two sites exceeding 2 kg m-2 (wet weight) in 2013; although that level was not maintained throughout the summer growth period. The invertebrate community was transformed under increased algal biomass within an increase in abundance (m-2) of smaller less energy-dense species when algae reached ~800 g m-2 (wet weight). Variation in overall invertebrate community assemblage between benthic samples was best explained by algae biomass; either singularly or in combination with % organic matter. This pattern was repeated with an initial increase in available energy (kJ m-2) within each bird species preferred prey under lower macroalgal mat biomass (~800 g m-2 wet weight) followed by a decline as algae biomass increased. During autumn, when large areas of macroalgal mat were still visible, foraging behaviour by some wading bird species varied under different levels of algae coverage. Some wading bird species’ behaviour also varied in winter on areas which had been covered by algae during the previous growth season. The current macroalgal mat biomass threshold of concern under SSSI Conditions Assessments is 2 kg m-2 (wet weight). Results suggest that the impact from macroalgal mats in Poole Harbour is evident at a biomass lower than 2 kg m-2 (wet weight); supporting a lowering of this threshold to 1 kg m-2. An increase in abundance of smaller invertebrates has resulted in a decline in energy available for the wading birds’ preferred diet. Observations suggest that wading birds may be adapting to these changes with some species appearing to be feeding on smaller, lower-quality prey (i.e. smaller worms/bivalves) and other species feeding on prey found on the surface of the macroalgal mat. Adaptations are site-specific in response to conditions within individual bays but consistent between shore levels within each bay.