Changes in zooplankton community, and seston and zooplankton fatty acid profiles at the freshwater/saltwater interface of the Chowan River, North Carolina PeerJ

The variability in zooplankton fatty acid composition may be an indicator of larval fish habitat quality as fatty acids are linked to fish larval growth and survival. We sampled an anadromous fish nursery, the Chowan River, during spring of 2013 in order to determine how the seston fatty acid composition varied in comparison with the zooplankton community composition and fatty acid composition during the period of anadromous larval fish residency. The seston fatty acid profiles showed no distinct pattern in relation to sampling time or location. The mesozooplankton community composition varied spatially and the fatty acid profiles were typical of freshwater species in April. The Chowan River experienced a saltwater intrusion event during May, which resulted in brackish water species dominating the zooplankton community and the fatty acid profile showed an increase in polyunsaturated fatty acids (PUFA), in particular eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). The saltwater intrusion event was followed by an influx of freshwater due to high precipitation levels in June. The zooplankton community composition once again became dominated by freshwater species and the fatty acid profiles shifted to reflect this change; however, EPA levels remained high, particularly in the lower river. We found correlations between the seston, microzooplankton and mesozooplankton fatty acid compositions. Salinity was the main factor correlated to the observed pattern in species composition, and fatty acid changes in the mesozooplankton. These data suggest that anadromous fish nursery habitat likely experiences considerable spatial variability in fatty acid profiles of zooplankton prey and that are correlated to seston community composition and hydrodynamic changes. Our results also suggest that sufficient prey density as well as a diverse fatty acid composition is present in the Chowan River to support larval fish production.

浮游动物脂肪酸组成的变异可作为仔鱼栖息地质量的指示指标，因为脂肪酸与鱼类仔鱼的生长及存活密切相关。2013年春季，我们针对溯河洄游鱼类育幼场——乔万河（Chowan River）开展采样工作，旨在明确溯河洄游仔鱼栖息期内，悬浮颗粒物（seston）的脂肪酸组成与浮游动物群落组成、脂肪酸组成之间的变化关联。悬浮颗粒物的脂肪酸谱并未呈现出与采样时间或采样地点相关的显著分布模式。中型浮游动物（mesozooplankton）的群落组成存在空间差异，且4月的脂肪酸谱符合淡水物种的典型特征。5月乔万河发生海水入侵事件，导致半咸水物种主导浮游动物群落，此时脂肪酸谱中的多不饱和脂肪酸（polyunsaturated fatty acids, PUFA）含量上升，尤以二十碳五烯酸（eicosapentaenoic acid, EPA）和二十二碳六烯酸（docosahexaenoic acid, DHA）最为显著。海水入侵事件过后，6月的强降水带来了大量淡水径流。浮游动物群落再次以淡水物种为主，脂肪酸谱也随之发生相应变化；但二十碳五烯酸（EPA）的水平仍维持高位，在河流下游区域尤为突出。研究发现，悬浮颗粒物、微型浮游动物（microzooplankton）与中型浮游动物的脂肪酸组成之间存在相关性。盐度是与物种组成的观测模式及中型浮游动物脂肪酸变化相关的主要影响因素。本研究数据表明，溯河洄游鱼类的育幼栖息地中，作为仔鱼饵料的浮游动物脂肪酸组成存在显著的空间变异，且该变异与悬浮颗粒物群落组成及水动力变化密切相关。研究结果同时显示，乔万河具备充足的饵料生物密度与多样的脂肪酸组成，能够支撑仔鱼的生长繁育。