Limnology of shallow lakes of the semi-arid Central Pampa of Argentina

Abstract Aim The Central Pampa of Argentina has three recognized phytogeographic regions that arise due to the decrease in rainfall towards the west. The area has numerous lakes that are mainly temporary, with hydroperiods that relate to climatic cycles, although some of them have changed due to anthropogenic influence. Some of these lakes have been studied with special reference to zooplankton, but information on their physical and chemical aspects is scarce. Consequently, managing and evaluating the anthropogenic effects on these ecosystems is challenging. The objective of this study was to explore the limnological characteristics of lakes in different regions in the Central Pampa that experience different anthropic influences. Methods Ten lakes were sampled seasonally (January, April, July, and October) during 2007. In situ measurements included transparency, pH, temperature, and dissolved oxygen concentration, and 2-L water samples were collected to determine salinity, ion composition, suspended solids, nutrient concentration, and phytoplankton chlorophyll- a concentration. Results Salinity ranged from 0.32-136.72 g L-1, with Na+ being the dominant ion in nine lakes and Cl- and HCO3- predominating in the higher- and lower-salinity lakes respectively. Nutrient concentrations were high (total Kjeldahl nitrogen: 7.97-34.69 mg L-1; total phosphorous: 4.07-14.82 mg L-1), and all lakes were hypertrophic. We determined three lake classes: i) lakes transformed from low-salinity lakes into hypersaline ones through human inactivation of the fluvial system that fed it; ii) mesosaline temporary lakes lacking fish, with low concentrations of chlorophyll-a and influenced by agricultural activities, and iii) subsaline and hyposaline lakes, highly modified by urban sewage, converted in permanent lakes (which allowed fish fauna development) and with reduced water transparency (due to high concentrations of phytoplankton chlorophyll-a). Conclusions The chemical diversity of the studied lakes is low, and their predominance of Na+ and Cl- indicated that evaporation and crystallization control the water chemistry. Additionally, this study showed the consequences of the anthropic impact, which alter water chemical composition, trophic structure and, thus, the ecological characteristics of lakes.摘要 ## 研究目标 阿根廷中部潘帕斯草原存在3个已被公认的植物地理区（phytogeographic region），其形成源于向西降雨量逐渐递减。该区域分布有大量以临时性为主的湖泊，其水文周期（hydroperiod）与气候循环密切相关，但部分湖泊因人为活动影响已发生改变。此前已有部分针对该类湖泊浮游动物的研究，但关于其物理与化学特性的相关资料仍较为匮乏。因此，对这些湖泊生态系统的人为影响进行管理与评估颇具挑战。本研究旨在探究中部潘帕斯草原不同区域、受不同人为活动影响的湖泊的湖沼学特征（limnological characteristics）。 ## 研究方法 本研究于2007年按季节（1月、4月、7月及10月）对10个湖泊开展采样。原位（in situ）测定指标包括透明度、pH值、水温及溶解氧（dissolved oxygen）浓度；同时采集2升水样，用于测定盐度（salinity）、离子组成（ion composition）、悬浮固体（suspended solids）、营养盐浓度（nutrient concentration）以及浮游植物叶绿素a（phytoplankton chlorophyll-a）浓度。 ## 研究结果 本次采样的湖泊盐度范围为0.32~136.72 g·L⁻¹，其中9个湖泊以Na⁺为优势离子，高盐度与低盐度湖泊分别以Cl⁻和HCO₃⁻为优势阴离子。所有湖泊的营养盐浓度均处于较高水平（凯氏总氮（total Kjeldahl nitrogen）：7.97~34.69 mg·L⁻¹；总磷（total phosphorous）：4.07~14.82 mg·L⁻¹），且全部属于超富营养化（hypertrophic）湖泊。本研究将采样湖泊划分为3类：① 因人类阻断补给河道而由低盐湖泊演变为超盐湖泊（hypersaline lake）的水体；② 受农业活动影响的中盐度（mesosaline）临时性湖泊，该类湖泊无鱼类种群，叶绿素a浓度较低；③ 受城市污水（urban sewage）严重影响的亚低盐（subsaline and hyposaline）湖泊，此类湖泊已转变为永久性水体，支撑了鱼类群落的繁衍，且因浮游植物叶绿素a浓度过高导致水体透明度显著下降。 ## 研究结论 本次研究的湖泊水体化学多样性较低，其优势离子为Na⁺与Cl⁻，表明蒸发与结晶作用主导了水体化学过程。此外，本研究揭示了人为活动的影响后果：人为活动改变了湖泊水体的化学组成与营养结构（trophic structure），进而改变了湖泊的生态特性。