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：研究目的 阿根廷中部潘帕斯草原存在三个已被公认的植物地理区（phytogeographic region），其形成源于向西降水量逐渐递减。该区域分布有大量以暂时性为主的湖泊，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干扰已发生演变。此前已有部分针对该区域湖泊浮游动物（zooplankton）的专项研究，但有关其物理与化学特征的资料仍较为匮乏。因此，对这些生态系统的人为影响进行管理与评估仍存在较大挑战。本研究旨在探究中部潘帕斯草原不同植物地理区、受不同人为干扰的湖泊的湖沼学特征。 研究方法 本研究于2007年按季度（1月、4月、7月与10月）对10个湖泊进行采样。原位测定（in situ measurement）指标包括水体透明度、pH值、水温及溶解氧浓度；同时采集2升水样，用于分析盐度、离子组成、悬浮颗粒物、营养盐浓度以及浮游植物叶绿素a（chlorophyll-a）浓度。 研究结果 本次采样湖泊的盐度范围为0.32~136.72 g·L⁻¹，其中9个湖泊以Na+为优势阳离子，高盐度与低盐度湖泊的优势阴离子分别为Cl-与HCO3-。研究区域内营养盐浓度较高（凯氏总氮（total Kjeldahl nitrogen）：7.97~34.69 mg·L⁻¹；总磷（total phosphorus）：4.07~14.82 mg·L⁻¹），所有湖泊均处于超富营养化状态。本研究将采样湖泊划分为三类：i）通过人为阻断补给河流系统，由低盐湖泊演变为超盐湖泊的水体；ii）受农业活动影响、无鱼类栖息、叶绿素a浓度较低的暂时性中盐度（mesosaline）湖泊；iii）受城市污水严重干扰、已转变为永久性湖泊（可支撑鱼类群落繁衍）且因浮游植物叶绿素a浓度过高导致水体透明度下降的亚低盐（subsaline and hyposaline）湖泊。 研究结论 本次研究涉及的湖泊水体化学多样性较低，其以Na+与Cl-为优势离子的特征表明，蒸发与结晶作用主导了水体化学过程。此外，本研究揭示了人为干扰所带来的一系列后果：其改变了湖泊的水体化学组成与营养结构，进而影响了湖泊的生态特征。