Ingemar Ahlgren

Empirical and theoretical models of phosphorus loading, retention and concentration vs. lake trophic state

Models describing the dependence of lake trophic state on external phosphorus loading are of two general types: empirical, steady state models derived from statistical treatment of data from large numbers of lakes, and theoretical, dynamic models based on more or less detailed mathematical descriptions of kinetics of nutrient and population dynamics.In the former type of models, inlake P concentrations have been found to depend mainly on inflow P concentrations and on water residence time. Efforts to explain the remaining variance by other factors such as iron, calcium, alkalinity, etc. have generally not been successful. Relations between lake P concentrations and chlorophyll may be useful for the prediction of maximum chlorophyll levels with some confidence but large variations in the actual chlorophyll concentrations are caused by, e.g. zooplankton grazing, varying chlorophyll content of the algae and other limiting nutrients.Dynamic, theoretical models are more suited for the prediction of rates of response to perturbations than the empirical steady state models. Due to the much larger demand for input data and labour investment, these models are generally more suited for the scientist as tools for formalizing and testing hypotheses, than for the water manager. Simple models with flexible structure generally have advantages over complex models.Both empirical and theoretical models are potentially powerful tools in eutrophication research and management. It is important for both scientists and water managers to select the optimum complexity with regard to the problem to be solved and to the resources available in each case.

THE ETHIOPIAN RIFT-VALLEY LAKES - CHEMICAL CHARACTERISTICS OF A SALINITY ALKALINITY SERIES

The study on 10 lakes within the Ethiopian Rift Valley during March–May 1991 covered a range of conductivity (K25) between 286 and 49100 µS cm−1. HCO3− — COinf3sup2−and Na+ were the dominant ions in all the lakes. Concentrations of K+, Cl− and SOinf4sup2−increased with increasing salinity and alkalinity, whereas Ca2+ and Mg2+ decreased. Comparison of these data with previous records showed that a ten-fold dilution of total ionic concentration occurred over 30 years in Lake Metahara and about three-fold increase occurred over 65 years in Lake Abijata. Concentrations of soluble silica were generally high (12–222 mg SiO2 1−1) and increased with increasing salinity, except for Lake Chamo which showed SiO2 depletion (to < 1 mg SiO2 1 −1) over the past three decades.The relationship between ionic concentration and phosphorus was irregular although high phosphorus concentrations generally corresponded with increasing salinity. Fitting data to the Dillon & Rigler (1974) chlorophyll a — total phosphorus relationship suggested that lakes Zwai, Awassa and Chamo are phosphorus-limited, whereas others have surplus phosphorus.

Limnological studies of lake Norrviken, a eutrophicated Swedish lake

SummaryThe water chemistry, the nutrient budget and the phytoplankton and its production in Lake Norrviken, Central Sweden, was studied for one year in 1961–62. In this paper the chemical factors and the nutrient budget are presented. The phytoplankton and its production will be treated in a second paper.Lake Norrviken receives waste water from a yeast factory and sewage from communities involving about 1300 people. Heavy blooms of blue-green algae occur. The oxygen conditions in the lake are extremely unsatisfactory. Almost the entire water mass becomes oxygen-free during the winter under the ice-cover.The concentrations of nitrogen and phosphorus compounds in the lake water are high. The dominating form of nitrogen is ammonia and of phosphorus inorganic phosphate, except in surface water during the summer when the major part of nitrogen and phosphorus is bound in the plankton.The main source of the nutrient income to the lake is the waste water from the yeast factory, which is responsible for more than 80% of the nitrogen and 70% of the phosphorus.Only about 40% of the nitrogen and 50% of the phosphorus income to the lake leaves the lake through its outflow. The rest of the phosphorus is accumulated in the sediments but a large fraction of the nitrogen is presumed to undergo denitrification to free nitrogen. On the basis of the ratio of the content of nitrogen to phosphorus in surface sediment this fraction has been calculated to be about 37% of the total nitrogen income to the lake or 60% of the amount which does not leave the lake through the outflow.ZusammenfassungWasserchemie, Nährsalzbudget und Phytoplankton sowie dessen Produktion im See Norrviken, Mittelschweden, wurden 1961–62 während eines Jahres studiert. In diesem Aufsatz werden die chemischen Faktoren und das Nährsalzbudget behandelt. Das Phytoplankton und seine Produktion werden in einem zweiten Aufsatz dargestellt werden.Der See Norrviken empfängt Abwasser von einer Hefefabrik und kommunale Abwässer von etwa 1300 Personen. Kräftige Wasserblüten von Blaualgen treten auf. Die Sauerstoff-bedingungen im See sind ausserordentlich unbefriedigend. Fast die ganze Wassermasse wird während des Winters unter der Eisdecke sauerstofffrei.Die Konzentrationen der Stickstoff- und Phosphorverbindungen im Seewasser sind hoch. Die vorherrschende Form des Stickstoffs ist Ammoniak und die des Phosphors anorganisches Phosphat ausser im Oberflächenwasser während des Sommers, wo der grösste Teil des Stickstoffs und des Phosphors im Plankton gebunden ist.Die Hauptquelle für die Nährstoffzufuhr des Sees ist das Abwasser von der Hefefabrik, welches für über 80% des Stickstoffs und 70% des Phosphors verantwortlich ist.Nur etwa 40% der Stickstoff- und 50% der Phosphorzufuhr in den See verlassen den See durch den Ausfluss. Der Rest des Phosphors wird im Sediment akkumuliert, aber ein grosser Teil des Stickstoffs wird wahrscheinlich zu freiem Stickstoff denitrifiziert. Auf der Basis des Stickstoff/Phosphor-Verhältnisses in der Sedimentoberfläche lässt sich dieser Anteil auf 37% der totalen Stickstoffzufuhr in den See oder 60% derjenigen Menge, die den See nicht durch den Ausfluss verlässt, berechnen.

Microbial Response of a Freshwater Benthic Community to a Simulated Diatom Sedimentation Event: Interactive Effects of Benthic Fauna

A bstractThe response of a sediment microbial assemblage to a pulse of diatoms was studied over 36 days by measuring bacterial activity and biomass, ATP concentration, and overall community respiration in laboratory microcosms. Also, the contribution of macrofaunal chironomids to the decomposition of settling diatoms in benthic communities, and the relative importance of benthic meiofauna in community metabolism, were determined.The addition of diatoms resulted in an immediate response by sediment bacteria, with higher bacterial production recorded after only 2 h, and a more than tenfold increase within one day. The rapid response by sediment bacteria was accompanied by relatively high initial concentrations of dissolved organic carbon. In treatments receiving diatoms, higher bacterial production was sustained throughout the experiment. Surprisingly, neither these elevated production estimates, nor the starvation of controls affected bacterial abundance. Mean bacterial cell volume, however, was markedly affected by the addition of diatoms. Combining community respiration measurements and bacterial production estimates showed that growth efficiencies for sediment bacteria ranged from 14.6 to 34.5%. The contribution of ambient meiozoobenthos to carbon metabolism was less than 1%. Carbon budgets showed that 1.3 mg C was cooxidized along with 4.3 mg added diatom C.Sediment reworking by Chironomus larvae initially enhanced bacterial production, but the presence of Chironomus resulted in lower bacterial production estimates after 16 and 36 days. This was interpreted as a result of faster decomposition of diatoms in treatments with chironomids, which was validated by a faster decline of ATP and chlorophyll a in the sediment. Our results indicate that Chironomus larvae compete with sediment bacteria for available organic substrates.

Fatty acid quality of seston and its effects on small fish in the lakes Xolotlán and Cocibolca, Nicaragua.

Fatty acid quality of seston and its effects on small fish in the lakes Xolotlan and Cocibolca, Nicaragua.