Is thermal stability a factor that influences environmental heterogeneity and phytoplankton distribution in tropical lakes?

Abstract Aim To evaluate the effects of environmental heterogeneity promoted by thermal stratification on the distribution of limnologic variables and phytoplankton functional groups (FGs) in two natural lakes. Methods Monthly measurements were performed over a five-year period in the vertical profile of a warm monomitic shallow lake (Lago Carioca) and in a deep and meromictic (Lake Dom Helvécio). Results The vertical zonation generated by the high thermal stability during the stratification period promoted an increase in the spatial heterogeneity and, consequently, in the richness of functional groups of the two lakes. In the epilimnion, the dominance of small chlorophytes and desmids (NA, A and X1, ≤20μm) and larger dinophyceans (Lo, ≥20μm) were associated with high turbulence and light availability and soluble phosphorus limitation. In the metalimnion, the presence of filamentous cyanobacteria (R) and colonial chlorophyceans (F) of larger size (≥20µm) were associated with stable habitats with high concentrations of N-NH4 and P-PO4-3 and light limiting conditions. Comparatively, Lake Dom Helvécio presented a higher richness of FGs in the meta-hipolimnetic layers (SN, P) as well as a higher number of species per functional group. Seasonal changes in the climatic conditions (e.g. the decrease in air temperature with the consequent heat loss) caused the break of the water column stability, which promoted the redistribution of the dissolved nutrient forms and the increase of light limitation in the two lakes during the mixing period. Therefore, there was a drastic reduction in the richness and population biomass of FGs (≤80%). Conclusions Thermal stability and atelomixis were the main driving forces of vertical heterogeneity during the stratification, favoring the coexistence of FGs and, consequently, their increase in richness and biomass.

摘要 研究目的：评估热分层（thermal stratification）所催生的环境异质性，对两座天然湖泊内湖沼学变量与浮游植物功能群（FGs）分布的影响。研究方法：在为期5年的周期内，分别对一座暖单循环浅水湖泊（卡里奥卡湖，Lago Carioca）以及一座深水永分层（meromictic）湖泊（唐·埃尔韦西奥湖，Lake Dom Helvécio）的垂直剖面开展月度监测。研究结果：热分层时段内高热稳定性所催生的垂直分层格局，提升了两座湖泊的空间异质性，进而提升了其浮游植物功能群的丰富度。在表水层（epilimnion）中，小型绿藻与鼓藻（NA、A及X1类群，粒径≤20μm）以及大型甲藻（Lo类群，粒径≥20μm）的优势度，与高湍流、充足光照及可溶性磷限制条件相关。在变温层（metalimnion）中，丝状蓝藻（R类群）以及大型集落型绿藻（F类群，粒径≥20μm）的存在，与高浓度氨态氮（N-NH4）、正磷酸盐（P-PO4^3-）且光照受限的稳定生境相关。相较而言，唐·埃尔韦西奥湖的变温层-深水层（meta-hypolimnetic）中浮游植物功能群丰富度更高（SN、P类群），且每个功能群对应的物种数量也更多。气候条件的季节变化（例如气温下降导致水体热散失）会打破水体层结稳定性，进而在混合期内促使两座湖泊中溶解态营养盐形态发生重新分配，并加剧光照限制。在此背景下，浮游植物功能群的丰富度与种群生物量均出现大幅下降（降幅可达80%及以上）。研究结论：热稳定性与非完全混合（atelomixis）是热分层时段内垂直异质性的主要驱动因素，它们有利于浮游植物功能群的共存，进而提升了功能群的丰富度与生物量。