Can Humic Water Discharge Counteract Eutrophication in Coastal Waters?

A common and established view is that increased inputs of nutrients to the sea, for example via river flooding, will cause eutrophication and phytoplankton blooms in coastal areas. We here show that this concept may be questioned in certain scenarios. Climate change has been predicted to cause increased inflow of freshwater to coastal areas in northern Europe. River waters in these areas are often brown from the presence of high concentrations of allochthonous dissolved organic carbon (humic carbon), in addition to nitrogen and phosphorus. In this study we investigated whether increased inputs of humic carbon can change the structure and production of the pelagic food web in the recipient seawater. In a mesocosm experiment unfiltered seawater from the northern Baltic Sea was fertilized with inorganic nutrients and humic carbon (CNP), and only with inorganic nutrients (NP). The system responded differently to the humic carbon addition. In NP treatments bacterial, phytoplankton and zooplankton production increased and the systems turned net autotrophic, whereas the CNP-treatment only bacterial and zooplankton production increased driving the system to net heterotrophy. The size-structure of the food web showed large variations in the different treatments. In the enriched NP treatments the phytoplankton community was dominated by filamentous >20 µm algae, while in the CNP treatments the phytoplankton was dominated by picocyanobacteria

学界公认的主流观点认为，通过河流洪水等途径向海洋输入更多营养盐，会引发沿海海域的富营养化（eutrophication）与浮游植物（phytoplankton）水华（phytoplankton blooms）。本研究表明，在特定场景下这一认知或可被质疑。据预测，气候变化将导致北欧沿海海域的淡水输入量增加。该区域的河流水体通常呈褐色，这是因为除氮、磷之外，其还含有高浓度的外源溶解有机碳（allochthonous dissolved organic carbon，即腐殖碳（humic carbon））。本研究旨在探究腐殖碳（humic carbon）输入量的增加，是否会改变受纳海水的浮游食物网（pelagic food web）的结构与生产力。本研究依托中宇宙实验（mesocosm experiment）体系，以波罗的海（Baltic Sea）北部的未过滤海水为实验材料，设置两组处理：一组同时施加无机营养盐（inorganic nutrients）与腐殖碳（CNP组），另一组仅施加无机营养盐（NP组）。两组实验体系对腐殖碳添加的响应存在显著差异：NP组中，细菌、浮游植物与浮游动物（zooplankton）的生产力均有所提升，体系转为净自养（net autotrophic）状态；而CNP组仅细菌与浮游动物的生产力出现提升，使体系呈现净异养（net heterotrophy）特征。不同处理组的食物网粒径结构存在显著差异：在富营养化的NP组中，浮游植物群落以粒径大于20微米的丝状藻类为优势类群；而CNP组的浮游植物群落则由微微型蓝细菌（picocyanobacteria）占据优势。