Understanding spatio-temporal variability in the production of drift macroalgae is useful for monitoring and assessing the influence of local anthropogenic nutrient sources on coastal ecosystems. In this study we: (1) assessed how the drift macroalgal community composition and biomass have changed over a 22-year period in Port Phillip Bay (a large coastal embayment in Victoria, Australia); and (2) measured the amount of accumulated nitrogen in drift macroalgae and the contribution of anthropogenic nitrogen sources through isotopic analysis. The surveys revealed little overlap in species composition (<13%) and a significant decrease in standing stock biomass (from 631 to 49 g m^-2) between surveys (1995/1996 and 2017/2018). This was associated with a high degree of turnover (β-diversity) at the genus level between surveys (mean – 0.53), resulting in distinct communities at all sites, particularly among red algal genera. In addition, the occurrence of ephemeral macroalgal blooms and a semi-perennial mat of the red macroalga Botryocladia sonderi at 10-15 m depth at locations close to nutrient sources indicate drift macroalgae are important nutrient accumulators in the bay. Isotopic analysis revealed a high contribution of anthropogenic-derived nitrogen (δ¹⁵N > 17‰ and ~ 3% N content) at depths and locations close to nutrient sources, particularly at times of the year when large wastewater discharges occur. Harvesting of drift macroalgae could be a cost-effective solution for offsetting nitrogen inputs from anthropogenic sources, but it will be important to first investigate whether harvesting results in negative impacts to associated fauna or nutrient cycling.

了解大型漂移藻类生产的时空变化有助于监测和评估当地人为营养源对沿海生态系统的影响。在本研究中，我们：(1) 评估了菲利普港湾（澳大利亚维多利亚州的一个大型沿海海湾）22 年来漂流大型藻类群落组成和生物量的变化情况； (2)通过同位素分析测量大型漂移藻类中积累的氮量以及人为氮源的贡献。调查显示，调查期间（1995/1996 年和 2017/2018 年）物种组成几乎没有重叠（<13%），而立木生物量显着下降（从 631 至 49 gm ^-2 ）。这与调查之间属水平的高度更替（β多样性）有关（平均值 - 0.53），导致所有地点都有不同的群落，特别是在红藻属中。此外，在靠近营养源的 10-15 m 深度处出现了短暂的大型藻华和半多年生的红色大型藻Botryocladia sonderi垫，表明大型漂流藻是海湾中重要的养分积累者。同位素分析显示，在靠近营养源的深度和位置，人为氮的贡献很大（δ ¹⁵ N > 17‰，氮含量约 3%），特别是在一年中发生大量废水排放的时候。收获大型漂流藻类可能是抵消人为来源氮输入的一种经济有效的解决方案，但重要的是首先调查收获是否会对相关动物群或养分循环产生负面影响。