Dark accelerates dissolved inorganic phosphorus release of high-density cyanobacteria

Bloom-forming cyanobacteria dramatically influence nutrient cycling in eutrophic freshwater lakes. The phosphorus (P) assimilation and release of bloom-forming cyanobacteria significantly may also affect the phosphorus source and amounts in water. To understand the phosphorus release process of bloom-forming cyanobacteria below the accumulated surface and sedimentary bloom-forming cyanobacteria, the degradation of bloom-forming cyanobacteria dominated by Microcystis spp. at different cell density in the dark was investigated over a 25-day microcosm experiment. The dissolved inorganic phosphorus (DIP) and dissolved total phosphorus (DTP) contents increased with the increment of cyanobacterial density, and the dark status markedly increased the proportion of DIP in water during the decline period of bloom-forming cyanobacteria. Meanwhile, the process of cyanobacterial apoptosis accompanied by the changes of malondialdehyde (MDA) and phosphatase (AKP) contents, and the increases of superoxide dismutase (SOD) and catalase (CAT) activities of cyanobacteria in the dark, especially in low-density groups (5.23×108 cells L-1), which further affect the physicochemical water parameters. Moreover, the DIP release from high-density cyanobacteria (7.86×107 cells L-1~5.23×108 cells L-1) resulted from the relative abundance of organophosphorus degrading bacteria in the dark. Therefore, the fast decay of cyanobacteria in the dark could accelerate DIP release, the high DIP release amount from accumulated bloom-cyanobacteria provide adequate P quickly for the sustained growth of cyanobacteria.

Spatial Variations in the Abundance and Chemical Speciation of Phosphorus across the River–Sea Interface in the Northern Beibu Gulf

Water samples were collected to measure dissolved and particulate phosphorus species in order to examine the dynamics of phosphorus in the water column across the river–sea interface from the lower Dafengjiang River to the open Beibu Gulf. Dissolved inorganic phosphorus concentrations were as high as 0.90 ± 0.42 μM in river water but decreased dramatically to as low as 0.02 ± 0.01 μM in open coastal waters. Total dissolved phosphorus was largely measured in the form of dissolved inorganic phosphorus in river waters (58% ± 18%), whereas dissolved organic phosphorus became the predominant species (>90% on average) in open coastal waters. Total dissolved phosphorus was the dominant species, comprising 76% ± 16% of the total phosphorus, while total particulate phosphorus only comprised 24% ± 16% of the total phosphorus pool. Riverine inputs, physical and biological processes, and particulate phosphorus regeneration were the dominant factors responsible for the dynamic variations of phosphorus species in the study area. Based on a two-end-member mixing model, the biological uptake resulted in a dissolved inorganic phosphorus depletion of 0.12 ± 0.08 μM in the coastal surface water, whereas the replenishment of dissolved inorganic phosphorus in the lower river from particle P regeneration and release resulted in an increase (0.19 ± 0.22 μM) of dissolved inorganic phosphorus in the estuarine mixing region. The molar ratios of dissolved inorganic nitrogen to dissolved inorganic phosphorus and dissolved silicate to dissolved inorganic phosphorus in the open surface waters were >22, suggesting that, although the lower Dafengjiang River contained elevated concentrations of dissolved inorganic phosphorus, the northern Beibu Gulf was an overall P-limited coastal ecosystem.

Dissolved Oxygen and Nutrients in coastal waters impacted by the Strymon River plume, North Aegean Sea, Greece

Three hydrographic cruises were conducted during 1997-1999 in north Aegean Sea, in order to study the dissolved oxygen (DO) and nutrient distributions in Strymonikos Gulf impacted by the riverine waters of the Strymon River. The approaches used in this paper include a preliminary estimation of the nutrient load of Strymon River and the influence of the riverine waters upon the DO and nutrient distribution in Strymonikos Gulf. Nutrient levels recorded in Strymon River were similar with other international Rivers. The influence of Strymonas River on the DO and nutrient distributions was clearly detected during December 1997 and May 1998. On the contrary, during March 1999 the influence of Strymon River was not so clearly detected. High nutrient concentrations were recorded close to the mouth of the river. As mixing with offshore waters progressed, nutrient levels declined dramatically. Plots of atomic DIN:P (ratio of the total dissolved inorganic nitrogen to dissolved inorganic phosphorus), Si:P (ratio of dissolved silica to dissolved inorganic phosphorus) and Si:DIN ratios in Strymonikos Gulf, during the three sampling periods studied, indicated stoichiometric P-limitation in about 34%. Furthermore, the data indicated significant probable N-limitation (31.4%), while Si-limitation was not probable (2.9%). In particular, 31% of the data showed a defined P-limitation in December, whereas 30% of the data showed N-limitation. It is noteworthy that Si-limitation was not recorded. In May, the atomic DIN:P ratio indicated P-limitation in about 21% and N-limitation in about 46%. Si-limitation was not recorded. Finally, in March, the 51% of the data showed P-limitation, whereas only the 16% showed N-limitation and 8% showed Si-limitation.