Trophic gradients in a large‐river Delta: ecological structure determined by connectivity gradients in the Danube Delta (Romania)

Abstract

There are over 300 lakes interconnected by riverbranches and man-made canals in the Danube Delta (Romania). A multidisciplinary survey of these riverine lakes situated in large wetland complexes was made comprising hydrological modelling, remote sensing and monitoring of water quality, plankton, aquatic vegetation and fish communities. Between-lake differences in water quality and aquatic vegetation cover were inferred from satellite image. Based on channel and lake dimensions, hydrological characteristics of the lakes in the delta were determined: hydrological distance from a riverbranch, residence time and impact of reed water. Water-quality and biotic parameters (phyto-and zooplankton, submerged plants and fish) were sampled in a comparative survey in June 1997-1999 in subsets of the lakes. In a follow-up study in 2001-2002, seasonality of aquatic vegetation and fish were recorded.A clear distinction was found in three lake types: (1) inflow lakes at short distance to the river, with a high flushing rate, high load of suspended minerals but low chlorophyll concentrations, high cover of floating and submerged vegetation and dominance of eurytopic fish; (2) large, relatively deep lakes with moderately long residence time, high Potamogeton-cover that collapses during summer, and dominated by eurytopic fish; (3) smaller, isolated lakes at the longest distance from the river, with long residence time, of which the water quality is strongly influenced by water flows through large floating reedbeds ('blackwater'), with a high vegetation cover of Ceratophyllum demersum and Nitellopsis obtusa throughout the summer and a limnophilic fish community tolerant for low oxygen conditions. Aquatic vegetation showed a clear seasonality related to the interaction between plant development and light conditions. Seasonal changes in fish distribution within the system merely followed the vegetation trends in different lakes and the conditions in the river channels.Past trends in the trophic gradient are discussed in the context of hydromorphological modifications in the Delta. There is a high potential for restoring natural processes owing to the high connectivity and large scale of the system.

Organic matter governs N and P balance in Danube Delta lakes

“…2a, b). Although Danube water was supplied through the Crisan-Caraorman canal, backwash from extensive reedbeds loaded the Puiu-RosuRosult lake complex with plenty of organic substrates as previously observed by Coops et al (2008). The inorganic nutrient concentrations showed a clear decrease along the flow path in both lake complexes, except for the PuiuRosu-Rosult complex in the fall (Fig.…”

“…On average, 183 m 3 s -1 are discharged in the Sfantu Gheorghe (Bondar 2000), and water travel times to Lake Uzlina are *0.2 days (Oosterberg et al 2000). Both lakes also show high loads of suspended particles, and high cover of floating and submerged vegetation (Coops et al 2008). The other three Lakes, Puiu, Rosu, and Rosulet, are situated further distant to the Sulina branch and belong to the fluvio-marine delta.…”

“…1). This complex encompasses large and isolated lakes, where the water quality is also highly influenced by black water input from large floating reedbeds (Coops et al 2008). Lake Rosu is separated only by partially submerged sand bars, reed, and floating reed islands (Phragmites sp.…”

“…Seasonal differences may also be explained by changes in species composition. It was, for example, reported that cyanobacteria start to increase their biomass Lake Rosulet after June (Coops et al 2008), indicating reduced relative N availability in the fall.…”

Habitats and trophic relationships of Chironomidae insect larvae from the Sepotuba River basin, Pantanal of Mato Grosso, Brazil

“…Connectivity to floodplains increase allow an exchange of organic matter and biota between stream and riparian wetlands (Wantzen and Junk, 2000;Junk and Wantzen, 2004) and several components of hydrological connectivity can be distinguished in riverine systems (Wantzen and Junk, 2005;Coops et al, 2008). Within the Sepotuba River, the channels and streams show a lateral connectivity gradient of decreasing exchange between lake water and river water.…”