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Densely vegetated shallow lakes often experience low water column nutrient levels and reduced phytoplankton growth, but in some cases a high phytoplankton biomass can co-exist with submerged macrophytes. The conditions that favour phytoplankton blooms within areas colonized by submerged macrophytes remain largely unexplored. We investigated changes in water quality variables and phytoplankton community composition data in relation to macrophyte-induced thermal stratification in a shallow urban lake. The results indicate that submerged macrophytes may favour internal loadings of phosphorus and low DIN:TP ratios, in periods when macrophyte-induced thermal stratification of the water column is common. Blooms of nitrogen-fixing cyanobacteria occurred under the strongly stratified conditions triggered by the high macrophyte biomass and elevated canopy, even though nitrogen limitation was apparent during the whole growing season. These findings suggest that submerged macrophytes can promote blooms of nitrogen-fixing cyanobacteria once they are sufficiently tall and dense to induce stable water column conditions. [ABSTRACT FROM AUTHOR]

Assessing the potential for transfer or export of biogeochemicals or pollutants from catchments is of primary importance under changing land use and climatic conditions. Over the past decade the connectivity/disconnectivity dynamic of a catchment has been related to its potential to export material, however we continue to use multiple definitions of connectivity, and most have focused strongly on physical (hydrologicaly or hydraulic) connectivity. In this paper we use a dual-lens approach, where the dynamic balance between transport and reaction is constantly in focus, and define ecohydrological connectivity as the ability of matter and organisms to transfer within and between elements of the hydrologic cycle while undergoing biogeochemical transformation. The connectivity/disconnectivity dynamic must take into account the opportunity for a given reaction to occur during transport and/or isolation. Using this definition, we define three distinct regimes: (1) one which is ecohydrologically connected and diffusion dominated; (2) one which is ecohydrologically connected and advection dominated and (3) one which is both hydrologically and ecohydrologically disconnected. Within each regime we use a new non-dimensional number, NE, to compare exposure timescales with reactions timescales. NE is reaction-specific and allows the estimation of relevant spatial scales over which the reactions of interest are taking place. Case studies provide examples of how NE can be used to classify systems according to their sensitivity to shifts in hydrological regime, and gain insight into the biogeochemical processes that are signficant under the specified conditions. Finally, we explore the implications of this dual-lens framework for improved water management, for our understanding of biodiversity, resilience and biogeochemical competitiveness under specified conditions [ABSTRACT FROM AUTHOR]