The submerged vegetation of Lake Rotoma

Abstract

The Majority of Rotorua Lakes are rapidly becoming eutrophic and/or dominated by exotic hydrophytes. Lake Rotoma was studied to provide essential information on native aquatic hydrophytes within an oligotrophic lake, and to record the initial transition towards an exotic dominated community. The lake is of volcanic origin and has a markedly fluctuating water level because of the absence of a surface outlet. An underwater SCUBA investigation was made in 1972, followed by intensive sampling from 50 systematically located one transects in 1973. A suitable quadrat size and shape was selected and the sampling error was assessed. The species frequencies were calculated from presence or absence recordings. Quadrat cover, depth, gradient and substrate were also determined. Three characean species, Chara fibrosa f. acanthopitys. (A. Br.) R.D.W., Nitella leptostachys var. leonhardii (R.D.W.) R.D.W. and N.pseudoflabellata var. mucosa (Nordst.) Bailey, dominated the vegetation. The next most common pattern contained the same three species plus Lagarosiphon major (Ridley) Moss. The initial stages of L.major invasion in to a characean community was recorded from 1973 up to 1976. The charophytes ranged in depth from 0 - 17.5m and were located upon a wide variety of substrates and gradients. The native vascular plants were absent from many transects. The plants had a depth range from 0 - 4.5m with most occurring above 3.5m. The Low Mixed Community was primarily located in water less than 1.25ma at the N.E. end of the lake, which resulted in this area having a high species diversity. The exotic hydrophytes were more widespread and abundant than the native vascular phanerogams. The distribution of L.major and Elodea canadensis Michx. appeared to coincide with boating access, recreational usage and fallen submerged trees. The depth range was 0 - 6.0m, although the available habitats within this range have not yet been fully exploited. Emergent species were abundant within the S.W. inlet and also in the lagoons around the lake shoreline where sheltered conditions and shallow gradients prevail. An experimental study on the effect of erosion and sedimentation by removing the shallow water plant cover, showed that the presence of plants moderated the extent of erosion and also encouraged sedimentation. Recolonisation of cleared areas was discussed. Annual frequency changes in the vascular hydrophytes were largely influenced by water level fluctuations and by the extension of exotic species. Water-level fluctuations were observed to influence (a) the bottom limit of plant growth (probably indirectly through light penetration), (b) the upper limit of plant growth by determining available growing space, and (c) the rate of spread of exotic species by erosion and redistribution of existing stands. The 1973 raw data was processed and standardised in the form of species presence or absence, absolute frequency, and relative frequency within each transect. Distance matrices were constructed using non-Euclidean distance functions. A non-linear multidimensional scaling technique was used to construct transect and species ordinations. The advantages and problems associated with this technique are discussed and compared to alternative methods. That adopted was successfully tested for its ability to reconstruct a place map of the North Island of New Zealand. The configuration error was calculated for each ordination and was found to be less than for the alternative methods tested. From the first species ordination a concept of species prosperity was developed which involved the number of transects and quadrats within which each species was recorded, and the maximum depth recording for each species. Non-linear analysis indicated that average gradient might be a significant influence. From the first transect ordination, four vegetation groups were recognised and discussed. Depth, substrate, species dominance and gradient were significant within the ordinations. Non-linear analysis confirmed many of the previous interpretations of the ordinations but gave no further information.