Weisner S E B, Strand John A, Sandsten Håkan
Limnology, Lund University, Ecology Building, S-223 62 Lund, Sweden fax: +46 462224536; e-mail:
Oecologia. 1997 Feb;109(4):592-599. doi: 10.1007/s004420050121.
Shallow eutrophic lakes tend to be either in a turbid state dominated by phytoplankton or in a clear-water state dominated by submerged macrovegetation. Recent studies suggest that the low water turbidity in the clear-water state is maintained through direct and indirect effects of the submerged vegetation. This study examined what mechanisms may cause a recession of the submerged vegetation in the clear-water state, and thereby a switch to the turbid state. The spatial distribution of submerged vegetation biomass was investigated in two shallow eutrophic lakes in the clear-water state in southern Sweden. Biomass of submerged vegetation was positively correlated with water depth and wave exposure, which also were mutually correlated, suggesting that mechanisms hampering submerged vegetation were strongest at shallow and/or sheltered locations. The growth of Myriophyllum spicatum, planted in the same substrate and at the same water depth, was compared between sheltered and wave exposed sites in two lakes. After 6 weeks the plants were significantly smaller at the sheltered sites, where periphyton production was about 5 times higher than at the exposed sites. Exclosure experiments were conducted to evaluate the effects of waterfowl grazing on macrophyte biomass. Potamogeton pectinatus growth was decreased by grazing, whereas M. spicatum was not affected. The effects were greater at a sheltered than at a wave-exposed site, and also negatively related to distance from the reed belt. These results suggest that competition from epiphytes and waterfowl grazing hamper the development of submerged vegetation at sheltered and/or shallow locations. An increased strength of these mechanisms may cause a recession of submerged vegetation in shallow eutrophic lakes in the clear-water state and thereby a switch to the turbid state.
浅水富营养化湖泊往往处于以浮游植物为主导的浑浊状态，或者以沉水大型植物为主导的清水状态。最近的研究表明，清水状态下的低水体浊度是通过沉水植被的直接和间接作用得以维持的。本研究探讨了哪些机制可能导致清水状态下沉水植被衰退，进而导致向浑浊状态转变。在瑞典南部处于清水状态的两个浅水富营养化湖泊中，对沉水植被生物量的空间分布进行了调查。沉水植被生物量与水深和波浪暴露程度呈正相关，而水深和波浪暴露程度之间也相互关联，这表明阻碍沉水植被生长的机制在浅水和/或隐蔽位置最为强烈。在两个湖泊中，比较了种植在相同基质和相同水深的穗状狐尾藻在隐蔽和暴露于波浪的地点的生长情况。6周后，隐蔽地点的植株明显较小，那里的周丛生物产量比暴露地点高出约5倍。进行了围隔实验，以评估水禽放牧对大型植物生物量的影响。放牧使篦齿眼子菜的生长减少，而穗状狐尾藻则不受影响。在隐蔽地点的影响大于在暴露于波浪地点的影响，并且与距芦苇带的距离也呈负相关。这些结果表明，附生植物的竞争和水禽放牧阻碍了隐蔽和/或浅水区沉水植被的发育。这些机制强度的增加可能导致处于清水状态的浅水富营养化湖泊中沉水植被衰退，从而转向浑浊状态。
