可利用資源量對臺灣東部黑潮區植物性浮游生物體型大小結構季節性變化之影響

Abstract

本研究，探究海洋生態系中植物性浮游生物(phytoplankton; 以下簡稱植浮)之大小結構變動與環境因子時空變化之相關性。本研究中以體型大小頻譜之斜率作為植浮群集變動的指標，探究臺灣東部貧營養鹽之黑潮流域(Kuroshio)的表層海域中，植浮大小頻譜的時空變化。然而，在黑潮水域中的營養鹽濃度除冬季外，濃度非常低，原因是受到強烈的海水成層效應及生物快速利用營養鹽。因此，在本研究中使用植浮總生物量作為可用資源量指標。研究結果指出，黑潮水溫變動有明顯季節性變化，水溫與植浮大小頻譜斜率呈現負相關，與前人研究結果相異。可用資源量與植浮大小頻譜之相關性則隨季節變動而有所不同:小個體因有相對高的個體表面積與體積比值，吸收可用資源量的速度快於大個體，因此小生物個體先爆發；到夏秋兩季，由於環境中可用資源量已累積，且大個體贏得可用資源量競爭，因此大個體較佔優勢。但隨著溫度升高，可用資源量逐漸減少，大小頻譜之斜率也隨之下降，此時是依循著可用資源量與大小之相關性理論，當可用資源量減低時，小個體會越佔優勢；但到冬季，水溫再度變冷，可用資源量再次升高時，小個體生物會再次佔有優勢，大小頻譜斜率呈現較傾斜。簡而言之，我們發現在黑潮水域中，植浮大小頻譜之變動主要受到可用資源量之變動影響。

We examined the relationship between phytoplankton normalized biovolume size spectral (NBSS) slopes versus hydrographic variables in the Kuroshio region east of Taiwan and tested two classic hypotheses. First, phytoplankton size structure follows temperature-size relationship that the NBSS slopes become steeper with increasing temperature. Second, resource-size relationship prevails for phytoplankton, of which NBSS slopes become shallower with nutrient enrichment. Here, we used total biomass of phytoplankton community as a proxy for resource supply instead of more commonly used inorganic nutrients because inorganic nutrients are deplete and cannot be a reliable proxy for resource supply in oligotrophic oceans. Our results do not support the temperature-size relationship, as NBSS slopes became shallower with increasing temperature. In contrast, we found a positive relationship between NBSS slopes versus total biomass, generally supporting the resource-size relationship. The only exception occurred in spring, during which the NBSS slopes deviated downward from the regression. In spring, we observed that small cells were dominant as nutrient pulses in the Kuroshio region, which is contrast to expectation from the resource-size relationship. This discrepancy can be explained by the none-steady state theory, where in the initial stage of nutrient enrichments in oligotrophic regions in spring, that small cells with a high surface-area-to-volume ratio uptake nutrients faster than the larger ones. Later in summer and early autumn, NBSS slopes became shallower because large phytoplankton won the competition after nutrient enrichments and/or predators ate small phytoplankton. In autumn and winter, NBSS slopes became steeper again when nutrients were barren. In general, phytoplankton size structures in the Kuroshio east of Taiwan can be explained by the resource-size relationship; nevertheless, the none-equilibrium condition needs to be taken into consideration.