Changes in shore topography and in the characteristics of the main biological habitats were observed at some coastal sites of Moturoa / Rabbit Island over the survey years. These changes reflect a progressive trend of erosion of the high shore soil in the study area. Based on our field observations, these changes appeared to be associated with natural coastal dynamics, potential climate change effects (e.g., sea level rise, extreme weather events) and / or logging activities occurring on Rabbit Island rather than factors related to biosolids applications. Consistent changes in sediment composition were observed over successive surveys at some transects but comparisons of application and reference transects suggested that they are not related to biosolids application.
No symptoms of enrichment (e.g., excessive algal growth, sediment anoxia, hydrogen sulphide odours, etc.) were observed at most sites surveyed. However, evidence of enrichment in the form of relatively high macroalgal cover and / or potential anoxia in sediment profiles, was found at a few sites near biosolids application areas, and high macroalgal cover was found at both application and reference sites over the survey years. Blooms of macroalgae were found in localised areas throughout Waimea Inlet, particularly those with low sediment oxygenation and in muddy, sulphide-rich sediments, and including areas away from the Island (Stevens et al. 2020). Macroalgae can grow rapidly on Rabbit Island shores (and elsewhere in the Inlet), particularly during early and late summer peak growing periods (Davidson and Moffat, 1990). Opportunistic taxa (e.g., Ulva sp. and A. chilense) can reach problem densities in estuaries under enriched conditions. In 2004, increased accumulations of macroalgae were found in the upper drainage channel at transect VI (application), which reduced with improved tidal flushing following the re-opening of the channel at the western end of the Island (Gillespie and Asher, 2004). Stevens and Robertson (2014) also reported a 50% increase in macroalgal cover in the wider Waimea Inlet since 1990.
Overall, the results of our study did not show consistent differences in sediment TN concentration between application and reference sites over the survey years. Furthermore, we found progressive increases in sediment TN concentrations at sites adjacent to areas that have not received biosolids. Concentrations of TN among estuaries in Aotearoa New Zealand range from 250–3,700 mg/kg (median 250 mg/kg, 75%ile 747 mg/kg) (Berthelsen et al. 2019). The maximum TN concentrations measured in the 2014 (1,900 mg/kg) and 2019 (1,600 mg/kg) surveys were relatively high compared with other estuaries. A suite of indicators for the condition of estuaries has been developed in Aotearoa New Zealand, including interim indicators for TN (Robertson et al., 2016). The indicators place estuarine sediments into bands associated with different levels of stress on sensitive infauna. Concentrations of TN from the 2008, 2041 and 2019 surveys were within bands indicating minor (250–1,000 mg/kg) or moderate (1,000–2,000 mg/kg) stress. Changes in TN tend to reflect changes in sediment composition over time. However, there was no pattern of change that would suggest an effect of biosolids application. Rather, the increases in mud and organic matter at some sites is likely to reflect the generally increasing muddiness of the Waimea Inlet over time (Robertson and Robertson, 2014).
Changes in the concentrations of As and some trace metals in sediments over time did not show patterns that might suggest that application of biosolids was causing an accumulation of any of these potential contaminants. For example, concentrations of Cu, Ni and Zn increased at reference transects in addition to some application transects. Concentrations of most metals were lower than the respective ANZG (2018) guidelines for the protection of aquatic life, the notable exceptions being Cr and Ni. These latter metals occur naturally at relatively high concentrations in coastal sediments in the Nelson region and derive from soils in the catchment (Forrest et al. 2007). Concentrations of Cr and Ni were elevated to a greater extent at transects IV (application) and (R)X, and (R)XI. These three transects lie close to the outflow of the Waimea River and likely receive the highest inputs of sediment and associated metals from the catchment. Cu and Zn are also ubiquitous contaminants around sites of human activity, entering the aquatic environment via stormwater runoff and frequently accumulating in sediments over time (Williamson and Morrisey, 2000).
The interpretation of changes in concentrations of Cd over consecutive surveys is problematic because values in 2014 and 2019 were consistently much lower than in 2008 and more consistent with other surveys of concentrations in sediments in the Waimea Inlet. For example, Cd concentrations in sediments at sites around and downstream of the Bell Island WWTP discharge in the eastern part of Waimea Inlet, sampled in 2016, were in the range < 0.01–0.03 mg/kg (Morrisey and Webb, 2016) and similar to those measured at the study transects in 2014 and 2019. The 2014 and 2019 results suggest that concentrations around the Island are similar to those in other parts of Waimea Inlet and that there is no evidence of an effect from the biosolids application on Cd concentrations.
Concentrations of FIB in shellfish were considerably lower in the 2019 survey than in the previous two surveys. Concentrations of E. coli were below the standard for shellfish growing areas in the ‘Approved’ status (230 E. coli/100 g) (Ministry for Primary Industries, 2018). It is not clear if this represents a general improvement in the microbiological quality of the shellfish at the study sites, because bacterial concentrations can vary widely between sites and even within a single day (Boehm, 2007). However, as per other types of contaminants, the bacterial results do not indicate an effect of the biosolids application on bacterial levels in the shellfish.
The infaunal surveys also provided no evidence of any adverse effects of the biosolids programme on the Island’s infaunal communities. Opportunistic polychaete worms indicative of moderately enriched sediments, such as H. filiformis and Prionospio sp., were recorded at both reference and application transects. These taxa also occur at other locations within the Inlet, in similar abundances to those in our study (Robertson & Robertson 2014). The mean taxa richness and abundance of infauna communities in the Moturoa / Rabbit Island surveys was also within range of those occurring in other Waimea Inlet locations and in estuaries throughout Aotearoa New Zealand (Robertson & Robertson 2014, Berthelsen et al. 2019), despite the local variability between the sites surveyed (some in embayments, others in headlands). The presence of polychaete taxa indicative of moderate enrichment at both application and reference transects, and the general similarity of infauna communities at both reference and potential impact transects, is not consistent with an effect of application of biosolids to land on Moturoa / Rabbit Island.
The higher abundance of P. australis at reference transects relative to application transects is consistent with an effect of application. However, Davidson and Moffat (1990) found that P. australis has limited tolerance to dilute seawater and fine sediments and Robertson et al. (2015) also found P. australis to be relatively intolerant to mud. Therefore, this result could have been due to sediment grain size distributions at these transects because the reference sites had lower percentages of silt and clay (Fig. 3).
Increasing muddiness and eutrophication of estuaries is occurring throughout Aotearoa New Zealand (MacDiarmid et al., 2012, Berthelsen et al., 2019) and worldwide (e.g., Lotze et al., 2006), largely in direct (e.g., catchment land-use change) or indirect (e.g., climate change) response to human activities.