RESUMEN
Anthropogenic pressures considerably affect coastal areas, increasing nitrogen and phosphorous loads that lead to eutrophication. Eutrophication sometimes results in hypoxic and/or anoxic conditions near the bottom water. Dissolved oxygen (DO) concentrations influence redox-sensitive nutrients, which can alter the benthic flux of nutrients. We retrieved sediment cores from two sites in the eastern and western parts of Elefsis Bay, a semi-enclosed area of the Eastern Mediterranean, Greece, during winter and summer. In the western part, seasonally hypoxic or anoxic conditions occurred. We analysed pore-water samples under normoxic, hypoxic and anoxic bottom water conditions to study the pore-water nutrient concentrations variability under different oxygen regimes. Ex situ incubation experiments were conducted at the site experiencing oxygen deficiency by manipulating the DO concentrations. The pore-water nutrient concentrations showed higher variability at the site experiencing oxygen deficiency. Notably, elevated ammonium concentrations were observed in the pore water during anoxic conditions, in the 2-20-cm sediment layer. However, the benthic fluxes of ammonium and phosphate at the 0-2-cm sediment layer were comparable under hypoxic and anoxic conditions. The results of the incubation experiments demonstrate a direct decrease in nitrate concentrations as the DO concentrations diminished in the overlying water. The incubations after re-oxygenating the overlying water show that phosphate was more efficiently scavenged when anoxic conditions prevailed in the bottom water. The incubation experiments indicate the rapid response of the seafloor to oxygen availability, particularly concerning processes that influence nitrate and phosphate concentrations. These observations highlight the dynamic nature of nutrient cycling in shallow, seasonally anoxic environments, such as Elefsis Bay, and emphasise the sensitivity of the seafloor ecosystem to changes in bottom water oxygen availability.
RESUMEN
We studied the origin of elements of an undisturbed stream basin during the dry season as derived by atmospheric inputs and lithological processes. Α mass balance model was applied taking into account atmospheric (rain and vapor) inputs and their origin from marine aerosol and dust, as well as the contribution of rock mineral weathering and dissolution of soluble salts. The model results were enhanced using element enrichment factors, element ratios and water stable isotopes. Weathering and dissolution of bedrock and soil minerals contributed the main element portions, besides sodium and sulfate that chiefly derived from wet deposition. Vapor was shown to contribute water to inland waters of the basin. However, rain was the main source of elements compared to vapor, with marine aerosol being the only atmospheric chloride source, contributing also over 60 % of atmospheric sodium and magnesium. Silicate derived from mineral weathering (mainly plagioclase and amorphous silica), while soluble salt dissolution contributed the main portions of the rest of major elements. In headwater springs and streams, element concentrations were more affected by atmospheric inputs and silicate mineral weathering was more intense, contrary to lowland waters that were more affected by soluble salt dissolution. Effective self-purification processes were mirrored in low nutrient levels, despite the significant inputs from wet deposition, with rain being more important contributor than vapor for the majority of nutrient species. Relatively high nitrate concentrations in headwaters were attributed to increased mineralization and nitrification, while the downstream nitrate diminishing was due to prevailing denitrification processes. The ultimate goal of this study is to contribute in establishing stream elements' reference conditions using mass balance modeling approaches.
RESUMEN
Desiccation and re-flooding processes play a key role on hydrological features of non-perennial rivers. This study addresses the effects of these processes on the aquatic quality and unravels underlying biogeochemical processes of an intermittent river reach in southern Greece containing a spring-fed pool. Combined spatio-temporal sampling for physicochemical parameters, major ions and nutrients and high frequency automatic monitoring during a hydrological year (2010-2011) indicate that during the dry period, solute variation was controlled by "concentration" processes (i.e. evaporative concentration and high dissolved ion input from base flow sources). Metabolic and "concentration" processes appear intensified during desiccation and water temperature rise. Photosynthesis induced carbonate precipitation, while respiration increased with gradual desiccation, but did not cause carbonate dissolution. In certain cases, photosynthesis and respiration may have occurred simultaneously as a result of differing microhabitat metabolism within the same water body. However, during the entire desiccation cycle, autotrophic production exceeded respiration resulting in relatively high oxygen concentrations, even during the night. With increasing desiccation, a rise in nutrient assimilation occurred as well as ammonification and/or desorption of ammonium from sediments, with simultaneous loss of nitrate. During initial floods, flushing of carbonate phases was not significant. In contrast, initial flood events were characterized by the dissolution of very soluble salts, i.e. epsomite-type (MgSO4∗7H2O) and gypsum (CaSO4∗2H2O). Regarding sediment transport and nutrients, a 1000-times increase of suspended sediments was observed during re-flooding, while the nutrient quality degraded, particularly for N-species. Results of the current research may serve to better understand the links of hydrological and biogeochemical processes in non-perennial rivers and streams towards their efficient management and conservation.
