Loss rates from sedimentation, parasitism, and grazing during the growth, nutrient limitation, and dormancy of a diatom crop

Abstract

Factors controlling the growth and decline of a diatom crop were measured in well mixed Lough Neagh, Northern Ireland. Net population increase in spring was interrupted for a short period, due to stormy weather causing changes in the underwater light climate. The increase in chlorophyll ceased when soluble reactive phosphorus was depleted, while biomass continued to increase until silica reached undetectable levels. Losses due to washout (between 0.25 and 0.5% of the crop per day), parasitism (4% infestation by chytrids), and grazing by zooplankton (mainly Cyclops abyssorum Sars) were small in relation to the net population growth rate of 4% per day. Sedimentation rate showed a rapid increase during calm weather at the time of nutrient limitation, with over 90% of the population of Melosira italica (Ehr.) Kutz. and Stephanodiscus astraea (Ehr.) Grun reaching the sediment. Changes in internal morphology and a drop in the adenylate energy charge from 0.81 to 0.55 indicated that these cells formed resting stages. Decay was limited until 4 weeks later when, in association with increased activity of the benthic fauna, 75% of the chlorophyll a was consumed in the next 40 days. This resulted in only 1% of the population surviving through to the following year.