Colonial nesting waterbirds as vectors of nutrients to Lake Lesser Prespa (Greece)

The nutrients imported by breeding waterbirds should be considered when identifying the main sources of nutrient input to lakes. Lake Lesser Prespa (Greece), including the adjacent Vromolimni pond, hosts numerous protected waterbirds that nest in densely populated colonies across the reedbeds. The accelerated eutrophication of the lake in recent years has been of increasing concern. In addition to likely large sources of nutrients (i.e., anthropogenic activities, especially agriculture), nutrient input via waterbird excrement may further trigger eutrophication. We estimated the annual phosphorus (P) and nitrogen (N) input by the most abundant colonial-nesting waterbirds (great white pelican, Dalmatian pelican, great cormorant, and pygmy cormorant) into the lake and investigated their influence on water and sediment quality. Near the waterbird colonies, soluble nutrient concentrations in the lake sediments were higher, and chlorophyll measurements indicated higher algal growth near these sites in summer. Stable isotope analysis suggests that near the colonies, waterbirds are responsible for nutrient loadings that affect the lake sediment. The estimated N and P nutrient input into the lake by both pelican and cormorant species is at least 1243 and 1649 kg/yr, respectively. On a landscape scale, this level of loading could be of minor importance for the lake because N and P can reach 32.8 (SD 9.3) and 38.9 (5.8) mg/m2 per year, respectively. Locally, however, this level of loading might induce cyanobacterial blooms, illustrated by the analysis of isolated Vromolimni pond near the lake. Our findings emphasize the likely importance of nutrient loading by waterbirds for the lake system.

在甄别湖泊营养输入的主要来源时，需纳入繁殖水鸟所携带的营养负荷予以考量。希腊的普雷斯帕小湖（Lake Lesser Prespa）及其毗邻的弗罗利姆尼池塘（Vromolimni pond）栖息着大量受保护水鸟，这些鸟类在芦苇丛中形成高密度的集群繁殖群落。近年来该湖泊的加速富营养化问题愈发引人关注。除了已知的大型营养输入源（即人类活动，尤以农业活动为甚）外，水鸟粪便介导的营养输入可能进一步加剧富营养化进程。
本研究估算了区内数量最为丰富的集群繁殖水鸟——大白鹈鹕（great white pelican）、卷羽鹈鹕（Dalmatian pelican）、普通鸬鹚（great cormorant）与侏鸬鹚（pygmy cormorant）——每年向湖泊输入的磷（P）与氮（N）总量，并探究了它们对水体及沉积物质量的影响。在水鸟繁殖群落周边区域，湖泊沉积物中的可溶性营养物质浓度更高，夏季叶绿素检测结果也显示该区域的藻类生长更为旺盛。稳定同位素分析（stable isotope analysis）表明，在繁殖群落附近区域，水鸟是影响湖泊沉积物营养负荷的关键因素。
经估算，两类鹈鹕与两类鸬鹚向该湖泊输入的氮、磷总量分别至少达1243千克/年与1649千克/年。从流域尺度来看，这一负荷水平对整个湖泊而言影响相对有限——其氮、磷年输入通量分别仅为32.8（标准差9.3）与38.9（标准差5.8）毫克/平方米。但在局部区域，该负荷水平足以引发蓝藻水华（cyanobacterial blooms），这一结论可通过该湖泊附近孤立的弗罗利姆尼池塘的分析结果得到验证。
本研究结果凸显了水鸟介导的营养负荷对湖泊生态系统的潜在重要性。