Influence of agriculture and aquaculture activities on the response of autotrophic picoplankton in Laguna Macapule, Gulf of California (Mexico)

Introduction:The lagoon is a component of coastal zones,whose populations of autotrophic picoplankton(APP)remain largely unstudied.These lagoons display high-nutrient productivity and additionally may also be subjected to anthropogenic activities.This study selected Laguna Macapule,located on the eastern shore in the mid-region of the Gulf of California,due to the fact that a drainage network servicing the surrounding agricultural region(>230,000 hectares under cultivation)directs irrigation runoff,shrimp farm effluents,and urban wastewater containing large quantities of nutrients to be discharged into this lagoon.We propose to identify the APP’s response to various types of environmental and anthropogenic influence in this highly impacted coastal lagoon.Methods:Two sites(separated by 2.7 km)were monitored from December 2007 to December 2008.One,located at the entrance to Laguna Macapule(oceanic influence)and the other a discharge canal(eutrophic conditions)inside the lagoon at El Tortugón.Results:APP was the numerically dominant phytoplankton fraction(15×106 to 620×106 cells L−1)with coccoidal cyanobacteria as the dominant fraction throughout the year.Peak levels were reached in spring-early autumn and they were the second largest contributor to biomass.Abundance of APP cells corresponds to the lagoon’s eutrophic status.Maximum numbers and a higher average of APP were recorded at the El Tortugón channel during the warm season(months with SST higher than 24°C).The general positive relationship of the APP’s annual cycle at both sites as well as a negative relationship with heterotrophic nanoflagellates(HNF)abundance,supports the idea that natural forcing,in particular sea surface temperature(SST)is the predominant influences on APP’s seasonal variability.Conclusions:Distinguishable significant differences in APP abundances and nutrients were recognizable between the two sites.The interplay of these variables contributed to lower densities of APP in winter and high densities in springearly autumn.N:P=~4 suggests that spring-early autumn abundance of the APP autotrophic component was sustained by urea from shrimp farm discharge water.Thus,a total nutrient-based approach is likely the most suitable tool for establishing nitrogen limitation of biological production in Laguna Macapule and similarly impacted ecosystems around the world.