Environmental heterogeneity and spatial distribution of macrophytes in plain streams

The spatial distribution of two biological groups of macrophytes (submerged and emergent) was studied in relation to the current velocity and stream morphometric variables (width, depth and width/wetted perimeter ratio) in three plain streams of the Buenos Aires province, Argentina. The frequency of the different species along 40 transects crossing the streams was determined. The most commonly found species were Egeria densa Planch, among the submerged plants and Rorippa nasturtium-aquaticum (L.) Hayek, Hydrocotyle ranunculoides L. and Eleocharis dunensis Kuk. among the emergent ones. In spite of the narrow range of current velocities found (0.3-39 cm/s), the two biological groups were mainly found in the lower current velocity zones, especially the submerged species. The pattern of distribution in relation to the stream width, depth and width/wetted perimeter ratio was not as evident as that related to the current velocity. The submerged species were very scarce in narrow and shallow reaches, while the distribution of the emergent species was more independent from these variables. Regarding the unbroken cross-sectional covers, the bigger ones corresponded to the submerged plants; instead the emergent plants had a larger proportion of the smaller patches. The bigger patches (up to 1050cm) are found in zones where the current velocity is slower than 10 cm/s. Above this value, the patches do not exceed 210 cm of extension and belong to the emergent species only. These last ones do not form extensive patches, and remain constrained almost only to the banks of the streams. There is a general wide overlapping habitat, as shown by the Horn ' s index (0.85-0.89). The number of species per patch varied between 1 and 8, and particularly when the cross cover occupied the whole width of the stream only 5% had one species and the most frequent figure was four species. This can be related to the effects of the macrophytes as habitat modifiers, making that the positiveinteractions within the patches prevail over the negative interactions. (C) 2001 Elsevier Science B.V. Ah rights reserved.