ABSTRACT

Environmental indicators that reliably describe estuary trophic response to nutrient loading are required for the effective management of New Zealand estuaries. A common estuary eutrophication response is frequent, extensive and persistent opportunistic macroalgae blooms. This study explores the utility of the Opportunistic Macroalgal Blooming Tool (OMBT) in the New River Estuary (∼4600 ha), a shallow intertidal-dominated system within a pastoral farming catchment in Southland, New Zealand. Total nitrogen (TN) loads increased from 3206 T y⁻¹ in 2000 to 5143 T y⁻¹ in 2020. Concomitantly, areas of opportunistic macroalgae, measured using the OMBT, expanded across the available intertidal habitat (AIH) from 34.9 ha or 1.4% of the AIH to 1383 ha or 47.6% of the AIH, and the average wet weight biomass increased from 19.3 g m⁻² to a peak of 1326 g m⁻² in 2019. We hypothesise that opportunistic macroalgae expanded rapidly in direct response to temporal increases in TN load, with increases disproportionately contributed by expansion and intensification of dairy farming in the catchment. This study confirmed that the OMBT predictably responded to changes in TN concentration and may therefore be useful in defining estuary-specific TN concentration thresholds to inform likely load reductions required to achieve improved environmental outcomes.

摘要

新西兰河口的有效管理需要可靠地描述河口对营养负荷的营养反应的环境指标。常见的河口富营养化反应是频繁、广泛和持续的机会性大型藻类大量繁殖。本研究探讨了新河口（约 4600 公顷）机会性大型藻类开花工具 (OMBT) 的效用，这是新西兰南地一个牧区农业集水区中以浅潮间带为主的系统。总氮 (TN) 负荷从 2000 年的 3206 T y ^-1增加到 5143 T y ^-12020 年。与此同时，使用 OMBT 测量的机会性大型藻类面积从可用潮间带栖息地 (AIH) 的 34.9 公顷或 AIH 的 1.4% 扩大到 1383 公顷或 AIH 的 47.6%，平均湿重生物量增加从 19.3 gm ^-2到 2019 年的峰值 1326 gm ^-2。我们假设机会性大型藻类迅速扩张以直接响应 TN 负荷的暂时增加，而流域内奶牛养殖的扩张和集约化不成比例地促进了增长。该研究证实 OMBT 可预测地响应 TN 浓度的变化，因此可能有助于确定河口特定的 TN 浓度阈值，以告知实现改善环境结果所需的可能的负荷减少。