The annual cycle of growth rate and biomass change in Planktothrix spp. in Blelham Tarn, English Lake District

Abstract

SUMMARY 1. The changes in the vertical distributions of red coloured Planktothrix rubescens and green P. agardhii filaments in Blelham Tarn, English Lake District, were related to vertical profiles of temperature and light attenuation and to continuous records of the surface irradiance and windspeed, from August 1999 to October 2000. 2. The potential growth rate of each organism was calculated from the irradiance and temperature at 0.5 m depths and hourly intervals throughout the year, using algorithms determined from growth rates in culture. The analyses indicated that there was sufficient irradiance for growth, integrated over the 24‐h cycle, at depths down to the metalimnion where the Planktothrix populations stratified in summer. The compensation depth for growth by P. rubescens reached a maximum of 9.3 m in spring and midsummer, and fell to a minimum of 1.6 m in midwinter; the corresponding values for P. agardhii were 7.9 and 0.5 m. 3. The mixed depth (zm) exceeded the critical depth for growth (zb) by P. rubescens (the condition preventing population increase) on only 3 days of the year; for P. agardhii, however, zm exceeded zb on 31 days, contributing to its faster decline. The stratified population of P. rubescens was the major cause of light attenuation during the summer of 2000, and resulted in competitive exclusion of P. agardhii. 4. The calculated growth rates integrated over the depth of the water column in Blelham Tarn equalled, or exceeded, the measured changes of the populations during periods when they were increasing, during summer and autumn. Close agreement between the two values was found for much of the year when allowance was made for dilution of the lake population by rainfall over the watershed. During periods of rapid decline, of P. agardhii in September 1999, P. rubescens in December 1999 and both in July–August 2000, additional losses (e.g. by chytrid parasitism and grazing) are invoked.

Possible Implications of Chytrid Parasitism for Population Subdivision in Freshwater Cyanobacteria of the Genus Planktothrix

“…to be highly potent and specialized parasites with chemotype-dependent infectiousness. They possess all properties required to inflict significant mortality on host populations, which is in good agreement with previous field observations (8,11). Chytrid fungi may exert strong selective pressure on Planktothrix sp.…”

Variation in peptide net production and growth among strains of the toxic cyanobacteriumPlanktothrixspp.

“…The species P. rubescens and P. agardhii differ in pigmentation and planktonic life-form: P. rubescens is typically found in deep, stratified and oligo-to mesotrophic waters, while P. agardhii occurs in shallow and more eutrophic waters. Chromatic adaptation has not been observed within Planktothrix (Skulberg & Skulberg, 1985) and the hydrostatic pressure and the underwater light climate act as factors leading to the ecological divergence between both life forms (Beard et al, 2000;Davis et al, 2003). In some ecosystems both species have been reported to co-occur for years, for example in Blelham Tarn, United Kingdom (Davis et al, 2003) or in Lake Steinsfjorden, Norway (Halstvedt et al, 2007).…”

Isolation of a novel cyanophage infectious to the filamentous cyanobacterium Planktothrix agardhii (Cyanophyceae) from Lake Donghu, China

“…The filamentous cyanobacterium Planktothrix agardhii is one of the typical harmful bloom-causing cyanobacteria and can be commonly detected in a large range of aquatic environments (Scheffer et al 1997, Briand et al 2002, Davis et al 2003, including shallow eutrophic freshwater lakes and reservoirs used to provide drinking water. Some strains can produce hepatotoxic microcystins that are toxic to the health of animals including humans (Tonk et al 2005, Wiegand & Pflugmacher 2005.…”