ISSN:
1573-5125
Source:
Springer Online Journal Archives 1860-2000
Topics:
Biology
Notes:
Summary Artificial substrates are used for quantitative studies on production and patterns of benthic algae (HERDER-BROUWER, 1975; KLAPWIJK, 1978; and others). The method permits a quantitative approach by weighings or direct countings under the microscope. Such applications, however, imply suppositions on the homogeneity and representativity of the developed growths (TIPPETT, 1970). Observations in natural growths of algae (HILLEBRAND, 1977) indicate that spatial inhomogeneities (patterns) occur at different sampling scales. The present study deals with patterns at three levels on glass microscopy slides (26×76 mm) in floating perspex racks immersed in two eutrophic and rather unpolluted water bodies: the small pond Hortusvijver in Amsterdam (see HERDER-BROUWER, 1975) and the artificial lake Grote Maarsseveense Plas near Utrecht. The sampling levels included „micro”-distribution with 12 sectors on a single slide (3×4 in width/length partition), „meso”-distribution with 12 slides in a floating rack, and „macro”-distribution with a series of 10 racks. Micro- and mesodistribution were studied in the Hortusvijver, meso- and macrodistribution in the Grote Maarsseveense Plas. Countings of algae were made in randomly chosen fields of view under the microscope with known area. In most cases ca. 20 fields were counted at a magnification of 400x, covering ca. 1mm2 for each observational unit. Microdistribution was studied on slides immersed two weeks in August–September 1976 in the Hortusvijver. On a slide 5 sectors were chosen at random out of 12. In each of these sectors 10 countings were made in random chosen fields, resulting in 50 countings for a slide. Dominant and subdominant species (mostly diatoms) showed an aggregated (clustered) pattern. The less common, filamentous green algae and Chlorococcales occurred in a random pattern. Statistical evidence, however, of this random pattern is not possible, as the probability distribution known as negative binominal fits at low densities random as well as aggregated patterns. So it is possible that the random pattern (as it fits to a Poisson distribution) of these organisms is in fact aggregated with a low density. We concluded that the development of algal growth on a glass slide is unequal, so a sufficient number of countings is needed for a reliable description of algal density and pattern. There are great differences between glass slides within one rack (length 50 cm, mesodistribution). This was observed in winter in the Hortusvijver and in early spring (March 1977) in the Grote Maarsseveense Plas after three weeks of immersion. The dominant species (all diatoms) showed aggregation in comparisons between the glass slides, so it can be concluded that a single slide is not representative for a sampling station. Densities varied less within a glass slide than between the slides. Macrodistribution was studied in the Grote Maarsseveense Plas in March with ten racks of slides, 50 m apart from each other in a single series along the straight and relatively homogeneous SW bank with a reed vegetation. This location was chosen because of the apparent lack of environmental gradients. Even at high numbers of counted fields of vision aggregation occurred in the species with high densities (all diatoms). Differences between the racks proved to be greater than within the racks. A single rack is therefore not representative for a rather homogeneous waterbody. It was concluded that aggregation was highest at the level of macrodistribution and lowest at the level of microdistribution, but it occurred at all of the three levels and in all dominant species. Estimates of the production or the biomass of algae based on observations with artificial substrates are only reliable if the patterns of the involved (dominant) species are thoroughly described. High differences are observed between substrates from different locations in a apparently homogeneous environment, so studies of gradients are only relevant if the requirements of good quantitative work are fulfilled. Probably scraping the algae off the substrates and through suspension are required for the study of representative samples. As natural substrates are often less homogeneous than artificial ones, quantitative work with such substrates must be considered with even more caution.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/BF02284746
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