Net autotrophy in a fluvial lake: the relative role of phytoplankton and floating-leaved macrophytes

Abstract

This study combined water-and sediment flux measurements with mass balances of dissolved gas and inorganic matter to determine the importance of pelagic and benthic processes for whole-system metabolism in a eutrophic fluvial lake. Mass balances of dissolved O 2 , inorganic carbon (DIC), nitrogen (DIN), phosphorous (SRP), particulate N (PN) and P (PP) and Chl a were calculated at a nearly monthly frequency by means of repeated sampling at the lake inlet and outlet. Simultaneously, benthic fluxes of gas and nutrients, including denitrification rates, and the biomass of the dominant pleustophyte (Trapa natans) were measured, and fluxes of O 2 and CO 2 across the water-atmosphere interface were estimated from diel changes in outlet concentrations. On an annual scale, Middle Lake exhibited CO 2 supersaturation, averaging 313% (range 86-562%), but was autotrophic with a net O 2 production (6.35 ± 2.05 mol m -2 y -1 ), DIC consumption (-31.18 ± 18.77 mol m -2 y -1 ) and net export of Chl a downstream (8.38 ± 0.95 mol C m -2 y -1 ). Phytoplankton was the main driver of Middle Lake metabolism, with a net primary production estimated at 33.24 mol O 2 m -2 y -1 , corresponding to a sequestration of 4.18 and 0.26 mol m -2 y -1 of N and P, respectively. At peak biomass, T. natans covered about 18% of Middle Lake's surface and fixed 2.46, 0.17 and 0.02 mol m -2 of C, N and P, respectively. Surficial sediments were a sink for O 2 (-14.47 ± 0.65 mol O 2 m -2 y -1 ) and a source of DIC and NH 4 ? (18.84 ± 2.80 mol DIC m -2 y -1 and 0.83 ± 0.16 mol NH 4 ? m -2 y -1 ), and dissipated nitrate via denitrification (1.44 ± 0.11 mol NO 3 -m -2 y -1 ). Overall, nutrient uptake by primary producers and regeneration from sediments were a minor fraction of external loads. This work suggests that the creation of fluvial lakes can produce net autotrophic systems, with elevated rates of phytoplanktonic primary production, largely sustained by allochtonous nutrient inputs. These hypereutrophic aquatic bodies are net C sinks, although they simultaneously release CO 2 to the atmosphere.

Assessing Potential Algal Blooms in a Shallow Fluvial Lake by Combining Hydrodynamic Modelling and Remote-Sensed Images

“…), cyanophytes (e.g., Oscillatoria spp. ), and chlorophytes (e.g., Scenedesmus spp., Pediastrum simplex) [20]. Moving from 2007 to 2008, the Superior Lake registered a significant decrease in Chl-a concentrations due to the higher flow discharges, but always characterized by high diurnal variations [4,31].…”

“…Moreover, the Superior Lake is located just downstream of the "Valli del Mincio" wetlands Natural Reserve, which is also a Site of Community Importance. In this fluvial lake, allochtonous nutrient loads, generated upstream, largely exceed those recycled (or retained/lost) from sediments [20]. The water transparency average value is 1.1 ± 0.6 m (variations mainly due to particulate matter and phytoplankton growth), while Chl-a concentrations vary between a few and ~200 mg m Eutrophic to hypertrophic conditions result in dense phytoplankton communities, such as chlorophytes, euglenophytes, chrysophytes, diatoms and cyanophytes [4], that strongly limit available light to submersed macrophytes.…”

“…Water quality in terms of nutrient loading, risk of hypoxia and primary productivity is surely a function of water discharge [20,25], which was lower during the 2007 dry summer and typical of summer conditions in 2008 and 2011. In June and July 2007, high water temperature and scarce water renewal, together with a high stability of meteo-climatic conditions, determined hypoxic events on the lake bottom, due to high oxygen demand by the benthic and pelagic systems.…”

“…These lakes are shallow, nutrient-enriched, host different primary producers, and undergo pronounced interannual discharge variations, coinciding with seasonal water diversion for irrigation purposes and rainfall patterns. The interaction between the continuous import of nutrients and export of particulate matter, the hydrological regime and the lake morphology leads to a mosaic of environmental conditions, allowing the simultaneous occurrence of different macrophyte typologies and high phytoplankton biomass [20]. Bolpagni et al [21] found generally lower concentrations of chlorophyll-a (Chl-a) in the areas closest to the macrophyte stands compared to the outer zone and speculated allelopatic interactions or nutrient limitation.…”

Remote sensing of macrophyte morphological traits: Implications for the management of shallow lakes

“…and submerged macrophytes (mainly C. demersum) can seasonally colonize littoral zones and areas where the water flow is low. in the last few decades, the progressive eutrophication of the fluvial lake system has favoured a marked proliferation of opportunistic primary producers (phytoplankton and emerging or floating macrophytes) with effects also on the benthic system (e.g., lake infilling, (Pinardi et al, 2011). in particular, the development of invasive meadows of emerging and floatingleaved macrophytes (e.g., N. nucifera and T. natans) in periods of high water temperature, nutrient availability and stable weather conditions, has required management intervention by local authorities to control plant growth and to preserve ecosystem services, including water utility (Pinardi et al, 2011(Pinardi et al, , 2015.…”