Microbial Food-Web Drivers in Tropical Reservoirs

Abstract

Element cycling in aquatic systems is driven chiefly by planktonic processes, and the structure of the planktonic food web determines the efficiency of carbon transfer through trophic levels. However, few studies have comprehensively evaluated all planktonic food-web components in tropical regions. The aim of this study was to unravel the top-down controls (metazooplankton community structure), bottom-up controls (resource availability), and hydrologic (water residence time) and physical (temperature) variables that affect different components of the microbial food web (MFW) carbon stock in tropical reservoirs, through structural equation models (SEM). We conducted a field study in four deep Brazilian reservoirs (Balbina, Tucuruí, Três Marias, and Funil) with different trophic states (oligo-, meso-, and eutrophic). We found evidence of a high contribution of the MFW (up to 50% of total planktonic carbon), especially in the less-eutrophic reservoirs (Balbina and Tucuruí). Bottom-up and top-down effects assessed through SEM indicated negative interactions between soluble reactive phosphorus and phototrophic picoplankton (PPP), dissolved inorganic nitrogen, and heterotrophic nanoflagellates (HNF). Copepods positively affected ciliates, and cladocerans positively affected heterotrophic bacteria (HB) and PPP. Higher copepod/cladoceran ratios and an indirect positive effect of copepods on HB might strengthen HB-HNF coupling. We also found low values for the degree of uncoupling (D) and a low HNF/HB ratio compared with literature data (mostly from temperate regions). This study demonstrates the importance of evaluating the whole size spectrum (including microbial compartments) of the different planktonic compartments, in order to capture the complex carbon dynamics of tropical aquatic ecosystems.

Flood pulse regulation of bacterioplankton community composition in an Amazonian floodplain lake

“…This genus also had high relative abundances in other AFLs (Toyama et al, 2017), but not in rivers (Doherty et al, 2017;Toyama et al, 2017). High temperatures and the food-web structure of tropical aquatic environments enhance the contribution of these phototrophic picoplankton to primary production (Domingues et al, 2017;Sarmento, 2012). This is due to their higher surface:volume ratio compared to larger phytoplankton cells, which confers an advantage under the oligotrophic conditions common in Amazon waters (Lewis, 1976).…”

Estudo do fitoplâncton e bacterioplâncton no metabolismo do nitrogênio em reservatórios subtropicais

“…The amount of carbon associated with the cell volume of many PHY species is much greater than in HB and PPP (Rocha and Duncan 1985;Lee and Fuhrman, 1987;Zubkov et al, 1998). The average PHY biomass in Itupararanga and Lobo were similar to values observed in other subtropical meso-eutrophic reservoirs (Rangel et al, 2012;Domingues et al, 2016).…”

Insights into the factors influencing mercury concentrations in tropical reservoir sediments

“…In comparison to CDU, MBS and CUN, the catchments of FUN and FNS (where low THg concentrations were observed) have a high percentage of agricultural land cover (Table 1) 8,31,33,63 . For both reservoir systems, enriched δ 15 N signals were observed, indicating these sediments are influenced by the input of agricultural and/or wastewater runoff 64,65 .…”