Effects of brownification and temperature on benthic and pelagic primary producers in a shallow aquatic ecosystem

Background/Question/Methods

In shallow aquatic systems benthic and pelagic algae compete asymmetrically for nutrients and light. The asymmetry arises because pelagic algae attenuate light on the way to the benthic habitat, and benthic algae intercept sediment nutrients diffusing into the pelagic habitat. An increase in temperature and humic water loading (brownification) may affect this interaction, but experimental studies are required. To understand more the response of these primary producers to temperature and brownification we performed an experiment in a pond divided in 16 sections (11.5 x 6.7 m). We ran the experiment in the year 2012 where these two factors were manipulated in a factorial design with 4 replicates for each treatment. The sections were comprised with natural aquatic communities: zooplankton; pelagic and benthic algae; macroinvertebrates; macrophytes; and the fish three-spine stickleback (Gasterosteus aculeatus) as top consumer. Frames done with plankton net were let in the ponds on the sediment for 20 days to allow benthic algae colonization. We scraped the frames and took samples for benthic algal biomass determination. Water samples were taken for determination of phosphorus concentration and pelagic algal biomass. Oxygen loggers were incubated in situ to determine pelagic and benthic primary production. 

Results/Conclusions

Pelagic primary production and biomass responded positively to brownification. Benthic biomass did not respond to brownification, but benthic primary production was affected negatively by this factor. The decrease in benthic primary production may have been caused by a decrease in light supply, because of shading by pelagic algae and water color. This led to an increase in the water total phosphorus concentration, because the low benthic primary production intercepted less the flow of nutrients from the sediment to the water column. Consequently, pelagic algae were benefited from this nutrient enhancement. Temperature affected negatively benthic primary production, and pelagic primary production and biomass. Benthic primary production may have been constrained by macrophyte and/or grazers. Meanwhile, phosphorus limitation in the water column may have led to the decrease in both pelagic primary production and pelagic algae biomass, which may have suffered from metabolic constraints as well. No interaction effect was detected. Benthic and pelagic primary producers were affected similarly to the rise in temperature, but they differ in their response to brownification. Brownification of water bodies is predicted in future climate change scenario, and it is relevant to study its effect on aquatic systems since its effect can diverge for different habitats and can influence cross-habitat interaction.