Phytoplankton and Bacterioplankton Production and Trophic Relations in Lake Chapala

This chapter makes use of data from literature published during 15 years of research supported by NSF, USEPA, Baylor’s University Research Committee and Univ. Autonoma de Guadalajara’s Special Projects Office and Research Programs. A list of the literature can be found at the end of the chapter. This research has also been supported by the work of many graduate and undergraduate students and others interested in the conservation of Lake Chapala. The chapter integrates research related to production processes in Lake Chapala — primary and secondary production. In Lake Chapala average annual primary production has changed from 80 g C m−2 during 1983–84 to 100 g C m−2 during 1989–90. This change was due to a modification in the light climate in the eastern region of the lake. Production is governed by inorganic turbidity. Secchi transparency in the lake averaged 0.2 m in the most turbid region and 0.7 m in the least turbid region. Production is also affected by rainy and dry seasons, typical of the tropics. The lake becomes less turbid during the rainy season due to increased depth and lessened sediment resuspension. In laboratory bioassays and in-lake container assays using both artificial and native algae it was determined that the most limiting nutrient for the phytoplankton production is nitrogen, but the ultimate factor is the poor availability of light due to high clay turbidity. Phytoplankton chlorophyll a averaged 5.4 mg m−3 of the mixed water column for all regions in the period of 1983–84. Chlorophyll a at the shallowest and most turbid station averaged almost twice that of the other stations. Chlorophyll a increased through the rainy season (as transparency and inorganic N content increased). During 1989-90 chlorophyll values increased to an average of 13.9 mg m−3. Again this change is due to a change in the light climate at the eastern region of the lake. As inorganic turbidity (clays) becomes a limiting factor for the light-dependent phytoplankton production it also can play other roles in the Lake Chapala ecosystem. Suspended clay particles provide surfaces for the adsorption of dissolved organic material. Heterotrophic bacteria can use this concentrated form of organic matter. The clay-organic matter aggregates are colonized by the bacteria and > 80% of the planktobacteria are attached to these aggregates. Bacterial numbers increment as turbidity increases and their biomass is significantly greater in more turbid regions of the lake. Mean annual bacterioplankton production was 67 mg C m−3 day−1 during 1989–90. The bacterioplankton production showed little seasonal variation in the lake. During the same period the area-weighted annual total phytoplankton production of the entire lake was 12.7×1010 g C yr−1 and the bacterioplankton production, 9.3×1010 g C yr−1. Phytoplankton and bacterioplankton production are related. The suspended clay particles limit the phytoplankton production due to light reduction; this negatively impacts bacterioplankton production due to the reduction of organic-substrate supplied by phytoplankton. At the same time, clay facilitates bacterial production by concentrating dissolved organic matter that is used by the attached bacteria. The relationship between phytoplankton, bacterioplankton and suspended aggregates give Lake Chapala a unique food web that is in need of further investigation. The presence of the large clay-organic-bacteria aggregates and their relationship to the lake’s food web can have interesting and serious implications.