Impact of black swan grazing and anthropogenic contaminants on New Zealand seagrass meadows

Seagrass is the only marine angiosperm (flowering plant) and decline of seagrass meadows has been reported worldwide. The important ecological and economic values of this marine habitat make it essential to preserve. Seagrass meadows of New Zealand are composed of only one species, Zostera muelleri. They are mainly present in sheltered, intertidal sand flats in harbours. Seagrass decline has been recorded in many New Zealand estuaries during the last century. Grazing by the black swan (Cygnus atratus) and anthropogenic contaminants resulting from catchment development are postulated to contribute to seagrass meadow loss in New Zealand and these factors have been assessed in this study. The aim of this study was to quantify black swan grazing pressure and examine effects of potential anthropogenic contaminants (specifically nutrient enrichment, sedimentation and herbicide residues) on seagrass meadows in New Zealand. The impact of current levels of these anthropogenic contaminants on seagrass resilience to swan grazing was also assessed in situ. To quantify black swan grazing pressure on seagrass meadows and to evaluate the potential relationship between swan grazing pressure and seagrass standing stock, observations and measurements were conducted in Tauranga Harbour where more than a third of the seagrass meadows disappeared between 1959 to 1996. Observations have shown that black swans graze intensively on Zostera muelleri meadows during high tide and were most numerous in the harbour during autumn. Black swan grazing resulted in the formation of circular devegetated patches (0.28 m2), where 92 % of shoots, 25 % of roots and 99 % of rhizomes are removed. The consumption rate was estimated to be 394 g dry weight (DW) swan-1 day-1. Significant seagrass loss on an annual basis was detected at the most swan populated site, which also had the highest swan grazing pressure (19-20 % of biomass consumed annually). This suggests a potential threshold of 19-20 % of annual seagrass biomass removal above which black swan grazing may contribute to substantial damage on seagrass meadows. Applied at the harbour scale, where there is a total of ~29 km2 of seagrass meadows, this corresponds to a swan population (i.e. grazing pressure) > 4630 birds. To identify the potential environmental stressors affecting seagrass condition, two contrasting harbours (Tauranga and Aotea) were surveyed where a range of environmental parameters, including parameters indicative of anthropogenic contamination, and seagrass condition metrics were measured. The survey showed that many of the seagrass condition metrics were positively correlated with sediment porewater ammonium (NH4-N) concentrations suggesting that nitrogen availability regulates seagrass growth. The presence of higher porewater nitrate (NO3-N) concentrations was linked to higher proportions of urban and crop land use in subcatchments, these latter parameters being negatively correlated with light availability during high tide. In spite of this, current levels of nutrient enrichment were apparently not detrimental to seagrass condition with evidence of N-limited plant growth and abundant light availability. Herbicide compounds were detected in seagrass sediment and at consistently higher concentrations in Tauranga harbour, which is the harbour more exposed to developed land use. The seagrass photosynthetic potential was lower in more herbicide contaminated sediments, although there was no evidence of any impact on seagrass biomass or morphometry. In a final experiment, black swan grazing was simulated in situ at four sites in Tauranga Harbour differing in exposure to potential anthropogenic contaminants and environmental characteristics. Grazing was simulated by removing seagrass biomass from patches (0.25 m2) in the meadow equivalent to natural swan removals (see above). Two levels of grazing intensity were applied to these patches; ~100 % of biomass removed (high) and ~ 40% of biomass removed (low). Results showed that the cover and the biomass of the low grazing intensity patches were not significantly different to controls (0 % of biomass removed) during the entire experiment. For the high grazing intensity patches, although the cover was restored after 9 months, biomass recovery was incomplete with only 30 % of total biomass regenerated, after one year. This suggests that swan grazing at high intensity can cause long-lasting damage to seagrass meadows, and that > 3 years is probably needed for a devegetated patch to fully recover from grazing (assuming a constant plant regeneration rate). The resilience response was similar across all sites examined in this harbour, suggesting that the current magnitude and range of environmental variability and potential contaminant levels within the seagrass meadows do not affect the ability of seagrass to regenerate from grazing. In conclusion, from these results, I can not conclude that anthropogenic contaminant or black swan grazing pressure are by themselves responsible for seagrass decline in Tauranga Harbour. However, the study suggests that herbicide residues have the potential to detrimentally affect the seagrass photosynthetic activity as do high grazing pressure by the black swan on the seagrass meadow condition. Overall, this study contributes to the understanding of impacts of black swan grazing pressure combined with potential contaminant effects on seagrass ecosystems. It emphasizes the importance of herbivory in temperate seagrass systems, which it is often underestimated and highlights the seagrass habitat contamination due to coastal development. It provides crucial information that will assist resource management agencies to preserve this valuable natural ecosystem