Dissolved Inorganic Phosphorus Ratio Research Articles

Unveiling the eutrophication crisis: 20 years of nutrient development in Zhanjiang Bay, China

Coastal eutrophication is a major issue of marine pollution. The main factors controlling eutrophication must be identified to ensure effective marine environmental management according to the respective local conditions. Zhanjiang Bay (ZJB), located northwest of the South China Sea, is a semi-closed bay influenced by complex water flows and the development of surrounding cities. In this study, we investigated the development of nutrient concentrations and compositions in ZJB seawater over the past 20 years and the factors influencing eutrophication based on several field investigations from 2006 to 2022 and historical data. High concentrations of dissolved inorganic phosphorus (DIP) and dissolved inorganic nitrogen (DIN) were the main contributors to the severe long-term eutrophication in ZJB; however, light eutrophication was observed in the outer bay, primarily caused by chemical oxygen demand (COD) and DIP. The primary sources of COD and nutrients were riverine freshwater, sewage outfalls, mariculture and domestic effluents carried by rivers. Tidal effects diluted the nutrient concentrations in the bay with seawater from the outer bay, thereby playing a key role in nutrient redistribution. The DIN: DIP ratio of ZJB showed long-term nitrogen restriction and excess phosphorus, primarily owing to mariculture activities. Marine undertakings can exert various impacts on water quality. Eliminating illegal aquaculture and launching aquaculture tailwater treatment can improve water quality, whereas practices such as channel dredging may worsen it. This study demonstrates the intricate dynamics of the ZJB ecosystem and offers valuable insights for effective environmental management and conservation efforts.

Species-Specific Response of Corals to Imbalanced Ratios of Inorganic Nutrients.

Dissolved inorganic phosphorus (DIP) is a limiting nutrient in the physiology of scleractinian corals. Anthropogenic addition of dissolved inorganic nitrogen (DIN) to coastal reefs increases the seawater DIN:DIP ratio and further increases P limitation, which is detrimental to coral health. The effects of imbalanced DIN:DIP ratios on coral physiology require further investigation in coral species other than the most studied branching corals. Here we investigated the nutrient uptake rates, elemental tissue composition and physiology of a foliose stony coral, Turbinaria reniformis, and a soft coral, Sarcophyton glaucum, exposed to four different DIN: DIP ratios (0.5:0.2, 0.5:1, 3:0.2, 3:1). The results show that T. reniformis had high uptake rates of DIN and DIP, proportional to the seawater nutrient concentrations. DIN enrichment alone led to an increase in tissue N content, shifting the tissue N:P ratio towards P limitation. However, S. glaucum had 5 times lower uptake rates and only took up DIN when the seawater was simultaneously enriched with DIP. This double uptake of N and P did not alter tissue stoichiometry. This study allows us to better understand the susceptibility of corals to changes in the DIN:DIP ratio and predict how coral species will respond under eutrophic conditions in the reef.

Hindcasting harmful algal bloom risk due to land-based nutrient pollution in the Eastern Chinese coastal seas

Harmful algal blooms (HABs) have been increasing in frequency, areal extent and duration due to the large increase in nutrient inputs from land-based sources to coastal seas, and cause significant economic losses. In this study, we used the “watershed-coast-continuum” concept to explore the effects of land-based nutrient pollution on HAB development in the Eastern Chinese coastal seas (ECCS). Results from the coupling of a watershed nutrient model and a coast hydrodynamic-biogeochemical model show that between the 1980s and 2000s, the risk of diatom blooms and dinoflagellate blooms increased by 158% and 127%, respectively. The spatial expansion of HAB risk caused by dinoflagellates is larger than that of diatoms. The simulated suitability of the habitat for bloom of Aureococcus anophagefferens, a pico-plankton of non-diatom or dinoflagellate, in the Bohai Sea is consistent with observations spatially and temporally. To halt further nutrient accumulation in the ECCS, reductions of dissolved inorganic nitrogen (DIN) (16%) and dissolved inorganic phosphorus (DIP) (33%) loading are required. To improve the situation of distorted DIN:DIP ratios, even larger reductions of DIN are required, especially in the Bohai Sea. Our approach is a feasible way to predict the risk of HABs under the pressure of increasing anthropogenic nutrient pollution in coastal waters.

Nutrients across Time: Relationships with Climate, Hydrology, and Land Use in Four Rivers of the Pacific Northwest

AbstractUnderstanding the temporal dynamics and drivers of dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphorus (DIP) provides a critical link to better management of nutrient‐related impacts such as eutrophication and harmful algal blooms (HABs). DIN and DIP are a primary control on persistent eutrophication and HABs in the Pacific Northwest (PNW). An understudied phenomenon, this paper examines multi‐decadal trends in DIN and DIP concentrations and loads, and their relationships to climatic and hydrologic factors (e.g., stream and air temperature, discharge, precipitation) in the PNW. Dissolved constituents act as a broad sentinel of linkages between watershed and in‐stream mechanisms such as nitrification, denitrification, nutrient use efficiency, evapotranspiration, hydrologic connectivity, groundwater extraction, irrigation, and land uses. As opposed to the total N and P often used in individual, autochthonous, lentic systems, DIN and DIP are used here as measures of multiscaled processes in allochthonous lotic systems with diverse flow paths. Time‐series data from public agencies were used for up to 20 years in river outlets from the Willamette, Salmon, Spokane, and Yakima watersheds. Seasonal Mann Kendall (SMK) tests suggest significant decreasing multi‐decadal trends in DIN and DIP loads for three out of four watersheds (for DIN, SMK = −0.104; for DIP, SMK = −0.081, −0.181, and −0.213), significant decreasing trends in DIN concentrations for one of the four watersheds (SMK = −0.144), and significant decreasing trends in DIP concentrations in three of the four watersheds (SMK = −0.120, −0.135, and −0.157). Multivariate regressions found significant relationships for concentrations, loads, and ratios when regressed against stream and air temperatures, precipitation, and discharge (16 significant regressions, with adjusted R2 values between 0.016 and 0.65). Highlights of these regression results are as follows: (1) precipitation, discharge, and water and air temperatures help to explain DIN and DIP concentrations and loads, (2) changes in DIN concentrations are sensitive to more hydroclimatic variables than DIP concentrations, and (3) DIP concentrations are positively correlated with stream temperature while DIP loads are negatively correlated with stream temperature. Furthermore, seasonal changes in nutrients — and their potential to alter aquatic productivity during a year — has received little attention in the literature. Regressions established significant seasonality or monthly variation of DIN and DIP concentrations and loads in all four watersheds (20 significant regressions, with adjusted R2 values between 0.038–0.65). Nutrient thresholds of DIN (0.3–0.5 mg/L) and DIP (0.05–0.005 mg/L) concentrations were used to analyze N‐ and P‐limitation. P‐limitation is known to occur in lakes, and N‐limitation is known to occur in rivers. Surprisingly, except for one watershed (Salmon), nutrient concentrations for both DIN and DIP in all watersheds were shown to be above the limitation thresholds across multiple seasons. In certain situations, such as where significant decreasing trends continue, the DIN:DIP ratio suggests seasonal switching between N‐ and P‐limited could create ideal conditions for HABs. The findings of this study have important implications for water resource management issues such as agriculture, land use development, fish populations, timber harvests, water quality, and public health.

Large temporal changes in contributions of groundwater-borne nutrients to coastal waters off a volcanic island

We examined the contribution of submarine groundwater discharge (SGD) to nutrient budgets in Hwasun Bay, Jeju Island, Korea in August 2009, October 2014, and May 2015. The concentrations of dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphorus (DIP) in fresh groundwater were in the range of 285−716 μM and 2.3−3.2 μM, respectively, which were each 1−2 orders of magnitude higher than those in the bay seawater. The outer-bay seawater flowing into the bay was oligotrophic (2.9 ± 1.9 μM for DIN and 0.2 ± 0.3 μM for DIP). Nutrient budget calculations were performed for each season by accounting for submarine fresh groundwater discharge (SFGD) and water residence times. In August 2009 (DIN = 1.8 μM and DIN:DIP ratio = 4.6 for the outerbay water), DIN inputs from SFGD accounted for approximately 40% of the DIN inventory in the bay seawater. In October 2014 (DIN = 1.1 μM and DIP < 0.05 μM for the outer-bay water), DIP from SFGD accounted for approximately 100% of the DIP inventory in the bay seawater. In May 2015, mean concentrations of DIN and DIP in the bay seawater were 8.6 ± 12 μM and 0.11 ± 0.04 μM, respectively, with conservative behaviors in the bay seawater in association with excessive groundwater inputs. These results imply that SGD plays a critical but different role in nutrient budgets and stoichiometry in coastal waters off a volcanic island depending on open-ocean nutrient conditions.

Effects of Dissolved Organic Nitrogen on the Growth of Dominant Phytoplankton in the Southwestern Part of East Sea in Late Summer

We investigated the distribution of dissolved nutrients, phytoplankton community structure and utilization of nitrogen compounds by dominant species in the southwestern part of East Sea in September, 2014. Dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphorus (DIP) were lower in the surface layer, and concentrations were increased with depth. Dissolved organic nitrogen (DON) and dissolved organic phosphorus were the opposite of dissolved inorganic nutrients. Although DIN DIP ratio in all of the water masses was higher than Redfield ratio (16), DIN : DIP ratio in mixed layer was about 2, indicating that inorganic nitrogen is the limiting factor for the growth of phytoplankton. In particular, DON proportion in dissolved total nitrogen was about 88 % in the mixed layer where inorganic nitrogen is limiting factor. The dominant species Chaeotceros debilis and Prorocentrum minimum were able to grow using DIN as well as DON such as urea and amino acids. Therefore, DON utilization of phytoplankton may play a role as a survival strategy in the DIN-limited conditions of East Sea.

Contrasted Saharan dust events in LNLC environments: impact on nutrient dynamics and primary production

Abstract. The response of the phytoplanktonic community (primary production and algal biomass) to contrasted Saharan dust events (wet and dry deposition) was studied in the framework of the DUNE ("a DUst experiment in a low-Nutrient, low-chlorophyll Ecosystem") project. We simulated realistic dust deposition events (10 g m−2) into large mesocosms (52 m3). Three distinct dust addition experiments were conducted in June 2008 (DUNE-1-P: simulation of a wet deposition; DUNE-1-Q: simulation of a dry deposition) and 2010 (DUNE-2-R1 and DUNE-2-R2: simulation of two successive wet depositions) in the northwestern oligotrophic Mediterranean Sea. No changes in primary production (PP) and chlorophyll a concentrations (Chl a) were observed after a dry deposition event, while a wet deposition event resulted in a rapid (24 h after dust addition), strong (up to 2.4-fold) and long (at least a week in duration) increase in PP and Chl a. We show that, in addition to being a source of dissolved inorganic phosphorus (DIP), simulated wet deposition events were also a significant source of nitrate (NO3−) (net increases up to +9.8 μM NO3− at 0.1 m in depth) to the nutrient-depleted surface waters, due to cloud processes and mixing with anthropogenic species such as HNO3. The dry deposition event was shown to be a negligible source of NO3−. By transiently increasing DIP and NO3- concentrations in N–P starved surface waters, wet deposition of Saharan dust was able to relieve the potential N or NP co-limitation of the phytoplanktonic activity. Due to the higher input of NO3− relative to DIP, and taking into account the stimulation of the biological activity, a wet deposition event resulted in a strong increase in the NO3−/DIP ratio, from initially less than 6, to over 150 at the end of the DUNE-2-R1 experiment, suggesting a switch from an initial N or NP co-limitation towards a severe P limitation. We also show that the contribution of new production to PP strongly increased after wet dust deposition events, from initially 15% to 60–70% 24 h after seeding, indicating a switch from a regenerated-production based system to a new-production based system. DUNE experiments show that wet and dry dust deposition events induce contrasting responses of the phytoplanktonic community due to differences in the atmospheric supply of bioavailable new nutrients. Our results from original mesocosm experiments demonstrate that atmospheric dust wet deposition greatly influences primary productivity and algal biomass in LNLC environments through changes in the nutrient stocks, and alters the NO3−/DIP ratio, leading to a switch in the nutrient limitation of the phytoplanktonic activity.

Agricultural impact on the pelagic ecosystem of the small temporarily open/closed Seteni Estuary, South Africa

The encroachment of agriculture and human population is placing increasing levels of stress on estuarine ecosystems worldwide. The catchment of the Seteni Estuary, a small temporarily open/closed estuary in South Africa, has been under extensive sugar cultivation for over 60 years. The present study reports on the impact of agricultural practices on the structure and dynamics of its pelagic communities over a 1-year period, from April 2008 to March 2009. The physico-chemical characteristics of the system were strongly linked to seasonality, with the system exhibiting a distinct open- and closed-mouth phase in response to rainfall. Concentrations of dissolved inorganic nitrogen (DIN) were exceptionally high in comparison to concentrations of dissolved inorganic phosphorus (DIP), resulting in high DIN : DIP ratios. Despite this, microalgal biomass remained relatively low, exhibiting maximum values of 91.7 mg chl a m–2 and 18.0 mg chl a m–3 for microphytobenthos and phytoplankton, respectively. Zooplankton abundance and biomass were significantly higher during the dry phase, with Pseudodiaptomus hessei dominating the zooplankton community both in terms of abundance and biomass throughout the study period. The ichthyofaunal community was dominated by Ambassis ambassis and Myxus capensis in terms of abundance and biomass, respectively. Results suggest that the system remains functional, but there is potential for deterioration, should the catchment become more nutrient-rich through poor management or land-use changes.

Alkaline phosphatase activity in the western English Channel: Elevations induced by high summertime rainfall

Alkaline phosphatase activity (APA) was determined in bulk particulate material and in a single-cell (ELF) assay at station L4 in the western English Channel during the summer of 2007. Throughout this period, the UK experienced its heaviest summertime rainfall since records began in 1914; with the result that riverine run-off into coastal waters was also elevated relative to long-term averages. Between May and August 2007, three distinct periods of elevated river run-off were observed which resulted in salinity minima at L4 on days 141, 190 and 232. An extended period of high river run-off between days 170 and 210 was responsible for decreases in near-surface salinity at L4 from 35.2068 to a minimum on day 190 of 34.7422. This contributed to the development of haline stratification which supported the development of an intense bloom of the centric diatom Chaetoceros debelis, with maximum observed chlorophyll a concentration of 8.69 μg l −1. Minima in salinity, and maxima in chlorophyll concentration on day 190 were coincident with a peak in river-derived dissolved inorganic nitrogen (DIN) of 1.9 μmol l −1 which was >5 times greater than the summertime mean and 24 times the concentrations experienced at L4 on weeks immediately before and after. There was no accompanying increase in dissolved inorganic phosphorus (DIP), and the DIN:DIP ratio increased to 49. With the inherent phosphorus stress that this caused, rates of APA increased from <4 to 42.4 nmolP l −1 h −1. ELF analysis on day 197 identified two taxa actively expressing alkaline phosphatase: the dinoflagellate Prorocentrum micans and ciliate Tiarana sp.

일본 Kochi현 Uranouchi만의 와편모조류, Heterocapsa circularisquama (Dinophyceae) 적조발생에 대한 환경 고찰

To study the environmental features causing a bloom of the novel dinoflagellate Heterocapsa circularisquama (Dinophyceae), hydrographic and chemical aspects were measured in the Uranouchi Bay, Kochi Prefecture, Japan, from January to December, 1997. The cell density of H. circularisquama increased rapidly in early October, and dropped sharply in mid-October. Growth rate of H, circularisquama during bloom period appeared 1.50 division day under high water temperature (25) and salinity (32 psu) conditions. Althought the result from hydrographic aspect indicated good condition for their growth, dissolved inorganic phosphorus (DIP) concentration in surface layer before bloom formation was less than 0.70uM, which is lower than their half saturation constant(Ks). Dissolved inorganic nitrogen(DIN): DIP ratio was > 30, indicating potential P-limitation. However, before bloom formation period of H. circularisquama, DIP concentrations were high in bottom layer (> 4.0 uM). Some studies reported that H. circularisquama had the ability to migrate vertically and to utilize dissolved organic phosphorus (DOP). Thus, DIP in bottom layer might have been utilized by H. circularisquama for their growth.DOP might have weakly affected their growth because of low reactive DOP concentrations owing to low DOP concentration (ca. 0.39 uM). Thus, if nutrient condition of bottom layer in Uranuchi Bay is not improved, the outbreaks of H. circularisquama red tides may became an annual feature.

東京湾奥域における主要植物プランクトンとＤＩＮ：ＤＩＰ比等との相互関係

The relationships between the dominant phytoplankton and the water quality factors were examined at two stations in the inner part of Tokyo Bay.Monthly changes of water quality factors in the inner part of Tokyo Bay showed that DIN (dissolved inorganic nitrogen): DIP (dissolved inorganic phosphorus) ratio was high in February and March and low in October and November, while TN (total nitrogen): DIN ratio was low in January and February and high in August and September.The relationships between the dominant phytoplankton and the water quality factors were considerably variable and complex. Skeletonema costatum, Heterocapsa triquetra, Eucampia zodiacus, Chaetoceros didymum, and Chaetoceros radicans occurred in waters with comparatively high DIN:DIP ratio (>80), Ceratium fusus, Ceratium lineatum, Rhizosolenia fragilissima, and Leptocylindrus danicus did in waters with moderate DIN:DIP ratio, and Heterosigma akashiwo, Prorocentrum minimum, Gonyaulax verior, and Chaetoceros debile did in waters with comparatively low DIN:DIP ratio (<50). DIN:DIP ratio in waters where dominant phytoplankton occurred tended to decrease with increased TN×TP and WT.

大阪湾および播磨灘における主要植物プランクトンとＤＩＮ：ＤＩＰ比等との相互関係

The relationships between the dominant phytoplankton and the water quality factors were examined at seven stations in Osaka Bay and Harima-nada (1995-1996).Monthly changes of water quality factors in Osaka Bay and Harima-nada showed that DIN (dissolved inorganic nitrogen):DIP (dissolved inorganic phosphorus) and TP (total phosphorus):DIP ratios were high in May and June, while DIN:DIP ratio was low and TN (total nitrogen):DIN ratio was high in August and September.The relationships between the dominant phytoplankton and the water quality factors were considerably variable and complex. Skeletonema costatum, Pseudonitzschia spp., Rhizosolenia fragilissima, and Eucampia zodiacus occurred in waters with high DIN:DIP ratio (>100), Ceratium furca, Noctiluca scintillans, Prorocentrum micans, Leptocylindrus danicus, Gyrodinium spirals, Mesodinium rubrum, and Prorocentrum triestinum did with moderate DIN:DIP ratio, and Gymnodinium mikimotoi, Rhizosolenia setigera, Chaetoceros curvisetum, and C. affinis did with low DIN:DIP ratio (<10). DIN:DIP ratio in waters where dominant phytoplankton occurred increased with increased TN×TP, TN:TP, and TP:DIP ratios, whereas it did not decrease with increased TN:DIN ratio.

大阪湾および播磨灘におけるギムノディニウムの高密度発生と水質，気象要因との関係

Relatioship between the blooming of Gymnodinium mikimotoi and water quality or meteorological factors was examined in Osaka Bay and Harima-nada, referring to the Meteorological Agency's data.The comparison of the average of each water quality parameter with the dominant species among phytoplankton from June to August of 1995-1997 showed that G. mikimotoi dominantly occurred in the waters with high water temperature, salinity, TN:DIN (total nitrogen: dissolved inorganic nitrogen) and TP:DIP (total phosphorus: dissolved inorganic phosphorus) ratios, moderate TN×TP and DIN:DIP ratio, and low TN, TP, DIN, DIP, and DSi (dissolved silicate). The red tide of G. mikimotoi occurred from late July to mid August of 1995 with meteorological conditions of high precipitation, high air temperature, long sunshine duration, and weak wind velocity in July.These characteristics of water quality and meteorological factors suggested that the abnormal occurrence of G. mikimotoi was closely connected with the increase of dissolved organic matter by the sudden increase and following decomposition of phytoplankton and the capability of utilization of dissolved organic matters of G. mikimotoi.

琵琶湖北湖におけるウログレナの優占的発生と水質，気象要因等との関係

In the north basin of Lake Biwa, harmful dinoflagellate Uroglena americana occurred dominantly, causing bad-smelling tap water in late spring for several years. The relationships between the abundance of U. americana and water quality in the north basin and the meteorological conditions of Lake Biwa were examined. The changes of water quality in the north basin (Station A) of Lake Biwa in early May of 1980-1993 showed that the fluctuation of WT (water temperature), DIP (dissolved inorganic phosphorus), TN: TP (total nitrogen: total phosphorus), DIN:DIP (dissolved inorganic nitrogen: dissolved inorganic phosphorus), and TP:DIP ratios were remarkable. The comparison of each average water quality with regard to the dominant phytoplankton in early May of 1980-1993 showed that U. americana occurred in the waters with water qualities near the mean values, excepting comparatively high WT. The species also occurred in the years with meteorological conditions from mid April to early May close to the mean. These features of water quality and meteorological conditions suggested that the alga was a common species in early May and the control of the dominant occurrences resulted from abnormal water quality and meteorological conditions.

諏訪湖におけるアオコの発生とＤＯＮ：ＤＩＮ比等との関係

In 1992, Microcystis and Anabaena blooms occurred in Lake Suwa from 10 June to 10 October, and the first and second peaks of TN (total nitrogen): DIN (dissolved inorganic nitrogen) and DON (dissolved organic nitrogen): DIN ratios in the water were found at near 10 July and 10 September. Microcystis viridis and Microcystis aeruginosa occurred with high cell densities near the first peak, and M. viridis and Anabaena flos-aquae occurred with high cell densities near the second peak. With regard to TP (total phosphorus): DIP (dissolved inorganic phosphorus) and DOP (dissolved organic phosphorus): DIP ratios, the positive peaks of these ratios were found near 10 July and the negative peaks near 10 September. These results suggest that TN:DIN and DON:DIN ratios are important factors which control the occurrence of Microcystis and Anabaena blooms, and TP:DIP and DOP:DIP ratios affect the constituent species of these blooms. From these results, the relative increase of DON in water and inorganization of DON due to the symbiotic bacteria might have supplied DIN to Microcystis, as many bacteria with the same form were similarly found at the middle layer of mucilage slime of colonies of 3 species of genus Microcystis.

琵琶湖北湖諸港におけるアオコの異常発生機構

Late summer in 1994, Microcystis bloom (dominant species: Microcystis aeruginosa) occurred for the first fime at two harbors in the north basin of Lake Biwa. The limit values of TN (total nitrogen) and TP (total phosphorus) for the occurrence of Microcystis bloom were ca 30 and 3 μg at/l, respectively.By comparing with the water quality factors of the harbors where Microcystis bloom occurred and the factors of harbors where Melosira bloom (dominant species: Melosila granulata) did, it was showed that TN, TP, DIP (dissolved inorganic phosphorus) and TN:DIN (dissolved inorganic nitrogen) tended to be higher and TN:TP and DIN:DIP ratios tended to be lower in the harbors where Microcystis bloom occurred. As the cause of the bloom of Microcystis in the north basin of the Lake Biwa, it was suggested that the suitable environment conditions for the quick growth of Microcystis was brought by the continuation of hot and clear weather in 1994. The increase of DIP exudation from bottom sediments, the intense consumption of DIN by phytoplankton, the disappearance of DIN due to nitrification and denitrification near the surface of bottom sediments might cause the increase of DIP and TN:DIN ratio. Further, the relative increase of DON (dissolved organic nitrogen) and the inorganization of DON by the symbiotic bacteria might supplied DIN to Microcystis, as many bacteria with same form were found at the middle layer of mucilage slim of colonies of Microcystis.

Nutrients, dissolved gases and pH in the Vaal River at Balkfontein, South Africa

Seasonal variation and interrelationships between environmental variables and nutrients as well as dissolved gases and pH in the Vaal River at Balkfontein were investigated between 1985 and 1989. Photosynthesis and temperature are major factors that influence the concentration and dynamics of O 2 and CO 2 in the river water. The average daytime oxygen concentration in the river (9.3 mg1 −1 ) rendered the water supersaturated (115 %) which suggests that photosynthesis exceeded respiration and that the river at Balkfontein was autotrophic. Minimum oxygen concentrations and low pH values were recorded during flood periods, coinciding with high turbidity, high temperature and low algal biomass. Vaal River water, with relatively high pH levels (average = 8.1), is a well-buffered system (average alkalinity = 89 mg CaCO 3 1 −1 ) with only the HCO 3 - species in the CO 2 -HCO 3 -CO 3 system of quantitative importance. Total suspended solids (TSS) is an important transport agent for phosphorus and nitrogen. The phosphate phosphorus (PO 4 -P) concentration was low (average = 16 μg 1 −1 ), but total phosphorus (TP) concentration was high (average = 202 μg 1 −1 ). Theretore, to sustain high algal growth, the organisms must have depended on recycling mechanisms to make dissolved inorganic phosphorus (DIP) available from TP. The TN: TP ratio in the Vaal River was relatively low (average = 11), but the DIN: DIP ratio was relatively high (average = 57), suggesting the occurrence of phosphate limitation in the Vaal River. Factors governing fluctuations in NO 3 -N and NH 4 -N concentrations (and therefore DIN-DIP ratios) in the Vaal River were discharge, decomposition of organic matter and algal blooms. The SiO 2 -Si concentration follows the average monthly water temperature, 'and probably plays an important role in diatom blooms

琵琶湖における優占植物プランクトンとＤＩＮ：ＤＩＰ比等との相互関係

The relationships between the dominant phytoplankton and the factors of water quality were examined in Lake Biwa, referring to the Shiga Prefecture's data. In the lake, TN (total nitrogen): TP (total phosphorus) and DIN (dissolved inorganic nitrogen): DIP (dissolved inorganic phosphorus) ratios tended to be lower with increased WT (water temperature), and this was more remarkable for DIN: DIP ratios than TN: TP ratios. As one of the causes, it was suggested that TN: DIN ratios increased and DIN decreased with increased WT. The relationships between the dominant phytoplankton and the factors of water quality were considerably variable and complex. Uroglena americana and Asterionella formosa occurred in waters with low WT and TN: DIN ratios and with high DIN, TN: TP and DIN: DIP ratios. On the other hand, Microcystis aeruginosa and Anabaena affinis occurred in waters with high WT, TN: DIN, TP: DIP ratios and with low DIN, TN: TP and DIN: DIP ratios. These results suggest that the examination on the relationships between the dominant phytoplankton and the factors of water quality was one of the useful methods to clarify the distinctive features of phytoplankton and to forecast the occurrence of algal blooms.