Microbial - Planktonic foodweb dynamics of a eutrophic Area of Concern: Hamilton Harbour

Hamilton Harbour, located on the western end of Lake Ontario, has a long history of cultural eutrophication as well as industrial contamination. We explored the structure and function of the microbial – planktonic foodweb during the growing seasons (May–October) of 2004 and 2006 in order to consider the flow of autochthonous production from lower to higher trophic levels. Our analyses included microscope based assessments of bacteria, heterotrophic nanoflagellates, ciliates, phytoplankton and zooplankton as well as radioisotope based measurements of primary productivity and bacterial growth. While routine measures of total phosphorus (avg: 25–33 µg l−1) and chlorophyll a (avg: 12–15 µg l−1) were indicative of eutrophy, mean phytoplankton biomass in 2004 (2.0 g m−3) and 2006 (2.2 g m−3) suggested mesotrophic conditions. However, the appearance of algal blooms in the summer of 2006 was an obvious indicator of cultural eutrophication. With respect to the microbial – planktonic foodweb, the organic carbon pool increased from a mean of 757.5 mg C m−3 in 2004 to 1160.3 mg C m−3 in 2006 and this increase was almost evenly split between autotrophs (198.7 mg C m−3) and heterotrophs (204.1 mg C m−3). The increased autotrophic carbon is readily attributable to the observed algal blooms driven by warmer temperatures and higher concentrations of soluble reactive phosphorus. However, the increase in heterotrophic carbon, primarily heterotrophic nanoflagellates, was apparent from the earliest observations in 2006 and remained consistently high throughout the year. We hypothesize that the increased heterotrophic carbon was the consequence of increased allochthonous carbon being shunted through the microbial foodweb; the energy generated was not likely transferred to zooplankton and passed on to higher trophic levels. More research into the dynamics of allochthonous and autochthonous carbon in eutrophic environments is called for.坐落于安大略湖（Lake Ontario）西端的汉密尔顿港（Hamilton Harbour），长期面临人类活动诱导的富营养化与工业污染问题。本研究于2004年与2006年的生长季（5月至10月），针对微生物-浮游食物网（microbial-planktonic foodweb）的结构与功能展开探究，旨在考察自源性生产产物从低营养级向高营养级的传递过程。分析手段涵盖针对细菌、异养纳米鞭毛虫（heterotrophic nanoflagellates）、纤毛虫、浮游植物与浮游动物的镜检评估，以及初级生产力与细菌生长速率的放射性同位素测定。尽管总磷（平均浓度25~33 μg·L⁻¹）与叶绿素a（平均浓度12~15 μg·L⁻¹）的常规监测指标均指向富营养化状态，但2004年（2.0 g·m⁻³）与2006年（2.2 g·m⁻³）的浮游植物平均生物量却提示水体处于中营养化水平。不过2006年夏季爆发的藻华，仍是该水域存在人类活动型富营养化的明确佐证。就微生物-浮游食物网而言，有机碳库的平均储量从2004年的757.5 mg C·m⁻³升至2006年的1160.3 mg C·m⁻³，该增量几乎由自养生物（autotrophs）与异养生物（heterotrophs）均分，两者增量分别为198.7 mg C·m⁻³与204.1 mg C·m⁻³。自养生物碳储量的增加，可直接归因于2006年夏季由水温升高与可溶性活性磷（soluble reactive phosphorus）浓度提升所诱发的藻华事件。但异养生物碳储量的增长（主要来自异养纳米鞭毛虫）早在2006年的首批观测中便已显现，并在全年维持高位水平。本研究提出假说：异养生物碳储量的提升是外源碳（allochthonous carbon）大量分流进入微生物食物网的结果；该过程产生的能量大概率无法传递至浮游动物，进而无法向上输送至更高营养级。未来仍需针对富营养化环境中外源碳与自源性碳的动态变化开展更多研究。