Green in the Pristine? Extensive subtidal bloom of Microdictyon umbilicatum at Great Barrier Island, northern New Zealand

Abstract

Green tides, the proliferation of green macroalgae in shallow coastal waters, generally occur as a result of anthropogenic nutrient enrichment. However, green tides have also been reported in areas characterised by excellent water quality and little to no anthropogenic influence. With increasing frequency since 2008 large quantities of the green algae Microdictyon umbilicatum has washed ashore beaches of Tryphena Harbour, the largest settlement on Great Barrier Island. Great Barrier Island is a sparsely populated island in north-eastern New Zealand with high ecological ratings from the local Council and a catchment dominated by native forest. Despite this reputation, the relatively recent onset of extensive and recurring macroalgae blooms is troubling. It raises the questions addressed in this thesis as to whether these blooms are a recent but natural occurrence, or whether they have been promoted by anthropogenic nutrient enrichment within Tryphena Harbour. Prior to the research carried out for this thesis, M. umbilicatum had not been studied or observed in blooming form in New Zealand, the source of wash-up events was unknown and the culprit species not conclusively identified. Therefore, the objectives of this study were to identify the source populations, describe the seasonal and spatial distribution and investigate drivers and potential causes of these blooms. It was hypothesised that if nutrient enrichment is an important driver, then seawater nutrient levels and tissue-N of M. umbilicatum would be higher in Tryphena Harbour compared to surrounding areas where the bloom does not occur. Similarly, if M. umbilicatum were using an anthropogenic nitrogen source in Tryphena then tissue-δ15N signatures would differ from the surrounding coast. Initial surveys in Tryphena revealed extensive subtidal beds of unattached M. umbilicatum drifting in sandy habitats. Drift underwater video (DUV) transects were used to survey the extent and seasonal distribution of these beds in Tryphena Harbour over a sixteen month period (October 2012 – February 2014). To compare environmental conditions within the Harbour to sites outside the Harbour (to the north and south), seasonal monitoring of seawater nutrients, physico-chemical parameters and M. umbilicatum tissue-N and tissue-δ15N concentration was carried out at twelve sites along the south-west coast of the Island. Growth experiments were also carried out at these sites, across a depth gradient in Tryphena Harbour and at sites on the mainland to compare rates of growth and determine if the potential exists for the bloom to spread. Lastly, a series of lab-based growth experiments were carried out to gain a better understanding of the optimum conditions for M. umbilicatum growth. Extensive subtidal beds of M. umbilicatum, up to 35 cm thick, were found to be primarily restricted to Tryphena Harbour, covering approximately 2.82km2,, ranging from depths of 5 – 22 m and having an estimated biomass of 1.5 ± 0.2 kg wet weight m-2. The blooms were persistent throughout the study period showing large seasonal and some inter-annual variation. The estimated biomass for the entire Harbour was higher in summer (2208 ± 207.5 tonnes wet weight) compared to winter (239 ± 96 tonnes), and benthic percent cover was higher in 2013 (summer; 52.7 %) compared to 2014 (summer, 32.4 %). Seawater nutrient and tissue-N concentrations at all Great Barrier Island sites followed a typical pattern for temperate waters, with high concentrations in winter and low concentrations in summer. Seawater sampling from sites within and outside of the Harbour showed no indication of elevated nutrients at Tryphena. Tissue-δ15N signatures of M. umbilicatum were generally within the range expected for unimpacted coastal areas in New Zealand (6.6 – 8.8‰), but on two sampling occasions δ15N values were slightly above this range (spring 2012, 9.53 ‰ and spring 2013, 9.20 ‰). These elevated values are consistent with an isotopically heavy 15N source that had undergone denitrification. M. umbilicatum experiments in situ showed significantly lower growth-rates at sites within Tryphena Harbour compared to those to the south (t=7.66, p=<0.001). Highest growth-rates were significantly correlated with total inorganic nitrogen (TIN, p = 0.002), exposure (fetch, p = 0.024) and water clarity (Secchi depth reading, p = 0.014); these were primarily driven by the two southern sites that had the highest nutrient concentrations, were most exposed and had the clearest water. Growth-rates at Tryphena sites were higher compared to mainland sites, and the sites showed opposing trends with depth. Tryphena sites exhibited significantly higher growth-rates in deeper waters (F=37.9, p=0.025), while mainland sites showed higher growth at the shallower sites (F=410.4, p=0.031) during the October 2013 trial. This relationship appears to be driven by the difference in light attenuation coefficients between Tryphena (0.09) and the mainland (0.16), as light was found to have a direct relationship with M. umbilicatum growth rates in lab-based experiments. The results described in this thesis indicate that Tryphena Harbour's unique physical setting facilitates the bloom by providing the optimal physical conditions for M. umbilicatum to exhibit high growth rates. Furthermore, these results demonstrate that the addition of any nutrients, even in a relatively small amount, will exacerbate the situation. As such, management actions should be taken to reduce or eliminate any potential anthropogenic nutrient enrichment into the Harbour, e.g. incorporating regular seawater and algae tissue monitoring at stream tributaries and identifying all potential nutrient source(s). This research has greatly enhanced the understanding of the blooming species, M. umbilicatum, and its unique environment at Tryphena Harbour. However, the initial cause of the bloom is still unknown as are consequences and/or benefits of the persistent and extensive drifting algae masses on Tryphena's sandy substrate. Further research will be required to identify potential anthropogenic nutrient source(s) that may intermittently enter Tryphena Harbour.