Abstract
Abstract Aim To verify the vertical distribution of phosphorus in the water and to identify the predominant forms of P in the water column for understand the phosphorus dynamics in tropical semiarid reservoirs during a prolonged drought period. Methods Two reservoirs from the semiarid region of Rio Grande do Norte were analysed during the period from May 2015 to June 2016. Were analysed: Suspended solids (SS), chlorophyll a (Chl-a), dissolved oxygen (OD) and temperature. Vertical profiles were plotted for total phosphorus (PT), total dissolved phosphorus (PTD), particulate phosphorus (PP), dissolved organic phosphorus (POD) and soluble reactive phosphorus (FRS). Results The phosphorus values distributed in the water column were high for both reservoirs, presenting the highest values during the periods with lower depth. Gargalheiras presented greater predominance of PT and PP, while Cruzeta had the highest values of FRS. Chl-a and SS values were also consistent with phosphorus values: Chl-a was higher in Gargalheiras, while SS, mainly inorganic, were higher in Cruzeta. Gargalheiras presented anoxic conditions close to the sediment from May 2015 to December 2015, which may induce the release of phosphorus from the sediment to the water column. Values that are too high during the shallower months, especially in Cruzeta, may have been influenced by the release of P from sediment through wind resuspension. Conclusions The amounts and predominant types of phosphorus in the water column are of great importance to understand the phosphorus dynamics and will support restoration plans for the studied environments. In this study it was possible to verify that the reservoirs are susceptible to the release of P from the sediment due to the environmental conditions, mainly low depths, resuspension of the wind and anoxia in the hypolimnion.
Highlights
Eutrophication has been defined as the nutrient enrichment of waters which results in increased production and growth of algae and macrophytes, unbalancing the ecosystem diversity and causing the degeneration of water quality (Dodds et al, 2009)
PP can be seen as a key parameter in P limited ecosystems, because it is a function of total biomass, but, the PP is associated to other fraction of P in water (SRP and dissolved organic phosphorus (DOP)) (Teubner et al, 2003)
The volumes of the reservoirs remained at very low levels dropping sharply until the beginning of 2016
Summary
Eutrophication has been defined as the nutrient enrichment of waters which results in increased production and growth of algae and macrophytes, unbalancing the ecosystem diversity and causing the degeneration of water quality (Dodds et al, 2009). The control of eutrophication is mainly focused on phosphorus, as it can be decreased more to limit algal growth (Smith & Schindler, 2009). Phosphorus is considered a limiting nutrient and is present in freshwater systems in forms: soluble reactive phosphorus (SRP), dissolved organic phosphorus (DOP), and particulate phosphorus (PP) (Teubner et al, 2003). The sum of soluble reactive phosphorus (SRP) and dissolved organic phosphorus (DOP) is the total dissolved phosphorus (TDP). SRP is the primary dissolved form of phosphorus and is readily available to algae and aquatic plants (Golterman, 2004). The determination of P forms in water is important to understand their dynamics and relationships with other components of the aquatic system, such as phytoplankton, macrophytes and sediment, and understand the implications in the water quality
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