Clay turbidity: Regulation of phytoplankton production in a large, nutrient‐rich tropical lake

Data from five sampling stations for 23 dates in 1 yr show that annual phytoplankton production in Lake Chapala is low (80 g C m −2 ) and governed by high inorganic turbidity. In the most turbid region of the lake, Secchi transparency averaged 0.2 m and the vertical light (PAR) attenuation coefficient ( η ″) averaged 9.7 m −1 ; in the least turbid region, Secchi transparency averaged 0.7 m ( η ″ averaged 2.3 m −1 ). Phytoplankton at the shallowest and most turbid station were most productive per unit volume, while the least turbid station, with a deep circulating water column, had the lowest volume‐based production. There was a considerable rainy vs. dry season difference in water transparency and in production. The lake became less turbid during the rainy season due to increased depth and lessened sediment resuspension. The annual lakewide production C.V. was 34% with the greatest day‐to‐day variation during the rainy season. Phytoplankton Chl a averaged 5.4 mg m −3 of the mixed water column for all regions. Chl a at the shallowest and most turbid station averaged almost twice that of the other stations. Chl a increased through the rainy season (as transparency and inorganic N content increased). Variation in inorganic N accounted for the greatest variation in Chl a at each region.