Seasonal Partitioning of Primary Production and Biomass between Phytoplankton and Metaphyton in a Shallow Lake Agmon, Hula Valley (Israel)

Agmon is a small, shallow man-made lake (area: 1.1 km2; mean depth 0.45 m), excavated in the peat soils of the Hula Valley in northern Israel, that was filled with water in August 1994. We followed the seasonal variations in phytoplankton and metaphyton biomass, primary production and related environmental conditions between December 1995 and July 1997. Water temperature ranged between 9.5°C - 30.8°C; pH ranged between 7.2 - 8.6. The algae in Lake Agmon were characterized by seasonal alterations between summer-fall phytoplankton blooms and spring proliferation of benthic algal mats, with a winter clear-water phase. Chlorophyll a content in the water, as a measure of planktonic algal biomass, was low in winter (1.75 - 5 μg·L-1) and high in summer (>100 μg·L-1), when planktonic cyanobacteria (Microcystis spp.) bloomed. Metaphyton biomass varied between 3.5 and 970 g·dry·wt·m-2, with chlorophyll a content ranging from 5 - 701 mg·m-2. The dominant benthic algal genera were Spirogyra and Oedogonium in 1996 and Cladophora in 1997. Phytoplankton primary production was high in summer-fall, with a maximum of 1200 mg·O2·m-2·h-1. Benthic primary production was high from March till May, with a peak of 2173 mg·O2·m-2·h-1 in April 1997. The rate of benthic algal primary production was positively correlated to benthic chlorophyll a (r2 = 0.90). Diel measurements of water column dissolved oxygen (DO) concentration, conducted monthly from January to May 1997, revealed that DO concentration ranged from a nighttime minimum of 5.3 to a noon peak of 15.3 mg·L-1. Only during January to February, no significant changes in DO with depth were found, suggesting that at that time the water column was well mixed. The most salient feature of primary production in the lake was the seasonal partitioning between its benthic and planktonic components. This was most evident in the significant inverse relationship between benthic and planktonic primary productivity rates (r2 = 0.78).