Ecological studies on Potamogeton pectinatus L. I. General characteristics, biomass production and life cycles under field conditions

Abstract

In this first part of a series on the ecology of Potamogeton pectinatus L., some general characteristics of the species and its habitats are recorded. The morphological characteristics of this phenoplastic species vary considerably under different environmental conditions. The taxonomy of the species remains unclear and complicated. The biomass allocation between different plant parts in the course of the year varies considerably between populations. In a sheltered brackish habitat in The Netherlands P. pectinatus showed a perennial life cycle and produced a maximum standing crop of 1312.5 g dry weight (DW) m −2 (1062.3 g ash-free dry weight (AFDW) m −2). In a relatively large and wind-exposed freshwater habitat the period of growth was very short and the maximum standing crop reached only 73.2 g DW m −2 (49.0 g AFDW m −2). In the Laiwersmeer (The Netherlands) foraging by waterfowl reduced the maximum shoot standing crop of a P. pectinatus bed to 17% of that of an ungrazed bed. In the Camargue (France) regulation of the water level had a distinct effect on the biomass development of a P. pectinatus population. In the Camargue all populations studied showed a perennial life cycle, although in larger brackish waters the biomass decreased considerably during autumn and winter, probably because of waterfowl grazing. The maximum standing crop varied from 375.7 to 668.0 g DW m −2 (275.2–549.8 g m −2 AFDW) in different habitats in the Camargue. Data on populations in Finland and on populations of running waters in The Netherlands are also presented. All populations studied showed flowering and achene production under field conditions, except for those in running waters. Although sometimes a considerable achene production occurred (20.4 g DW m −2, 18.4 g AFDW, 3707 achenes m −2 in the Baisse Salée, Camargue) all populations reproduced mainly vegetatively. The main function of the achenes appears to be dispersal and long-term survival of the species. The tubers played a very important role in the survival of populations. Populations of brackish waters predominantly produced single tubers and only rarely double tubers (less than 2%). Freshwater populations produced 30–60% double tubers under field conditions. Although reproduction is generally vegetative, it is argued that distinct annual and perennial life cycles can be recognized. Different life cycles can be induced by various environmental conditions. Life-cycle characteristics of several aquatic macrophytes are reviewed and it is concluded that basic information on life cycles and reproductive strategies of many aquatic macrophyte species is still lacking.