Sedimentary materials from eroding bluffs, suspended solids in streams, and lake bottom sediments from Lakes Ontario and Erie were cultured with the alga Scenedesmus quadricauda (Turp.) de Brebisson in modified Rodhe's medium with the sediments as the sole source of P. P uptake by the algae was related to the amount of nonapatite inorganic phosphorus in the sediments. Apatite phosphorus was not used, and the bluff samples, in which over 90% of total P was in this form, did not support algal growth. The nonapatite inorganic P fraction was highly correlated with the amounts of inorganic phosphorus extracted by three standard techniques for estimating "available P" (extraction by NaOH and nitrilotriacetic acid solutions and by H-resin) and cell uptake equaled NaOH-extractable inorganic P in several instances. Uptake of P by the cells varied from 8 to 50% of total P and from 38 to 83% of nonapatite inorganic P when measured directly. Organic phosphorus in the sediments was not utilized by the algae. Percentage utilization of total P was in general highest when total P concentration in the sediments was itself high.

Lake Cajititlán is a small, shallow, subtropical lake located in an endorheic basin in western Mexico. It is characterized by a strong seasonality of climate with pronounced wet and dry seasons and has been classified as a hypereutrophic lake. This eutrophication was driven by improperly treated sewage discharges from four municipal wastewater treatment plants (WWTPs) and by excessive agricultural activities, including the overuse of fertilizers that reach the lake through surface runoff during the rainy season. This nutrient rich runoff has caused algal blooms, which have led to anoxic or hypoxic conditions, resulting in large-scale fish deaths that have occurred during or immediately after the rainy season. This study investigated the changes in the phytoplankton community in Lake Cajititlán during the rainy season and the association between these changes and the physicochemical water quality and environmental parameters measured in the lake’s basin. Planktothrix and Cylindrospermopsis were the dominant genera of the cyanobacterial community, while the Chlorophyceae, Chrysophyceae, and Trebouxiophyceae classes dominated the microalgae community. However, the results showed a significant temporal shift in the phytoplankton communities in Lake Cajititlán induced by the rainy season. The findings of this study suggest that significant climatic variations cause high seasonal surface runoff and rapid changes in the water quality (Chlorophyll-a, DO, NH4+, and NO3–) and in variations in the composition of the phytoplankton community. Finally, an alternation between phosphorus and nitrogen limitation was observed in Lake Cajititlán during the rainy season, clearly correlating to the presence of Planktothrix when the lake was limited by phosphorus and to the presence of Cylindrospermopsis when the lake was limited by nitrogen. The evidence presented in this study supports the idea that the death of fish in Lake Cajititlán could be mainly caused by anoxia, caused by rapid changes in water quality during the rainy season. Based on our review of the literature, this is the first study on the phytoplankton community in a subtropical lake during the rainy season using high throughput 16S rRNA and 18S rRNA amplicon sequencing.

When the gas vacuoles of Anabaena flos-aquae Br6b. ex Born. et Flah. are collapsed, the optical properties of the alga change. While this may suggest a light-shielding role, photosynthetic measurements indicate that intact gas vacuoles reduce the light falling on the thylakoids by only 4~, or tess. Intact gas vacuoles offer no protection against the lethal effects of ultraviolet light. When the alga is grown at high light intensity the gas vacuoles are fewer in number but are oriented peripherally in the cells. However, this does not markedly affect their light shielding efficiency. Spectrophotometric measurements carried out by others indicate a light shielding role by gas vacuoles in a non-planktonic blue-green alga, Nostoc muscorum Kiitz., but do not give a quantitative estimate of this effect. In Anabaena no definite evidence of light-shielding is obtained by such a method. All of the experiments described were conducted with dilute algal suspensions to investigate shielding effects in individual cells. Possible selfshading effects in dense suspensions and surface water blooms require further investigation.

SUMMARY In axenic Chlorella pyrenoidosa Chick cultures, extracellular release was linear with time, but plateau‐type curves were obtained in cultures with added bacteria. Initial rates of excretion were identical in both, systems. Kinetics of extracellular release in axenic Anabaena flos‐aquae (Lyng.) Bréb. cultures were more complex than in Chlorella but the initial excretion rates were identical in axenic and mixed algal‐bacterial cultures. In lakewater, extracellular release kinetics resemble the pattern in mixed Chlorella‐bacteria cultures. An explanation is an initial lag in bacterial utilization of algal extracellular products. As a result, both in situ and in the laboratory, consecutive short, experiments give higher excretion rates than single long incubations. It is suggested that the former are close to total or gross extracellular release rates whereas the latter give net values, detecting only substances not, removed by heterotrophs.

We calculated the annual production of epilimnetic crustacean zooplankton in Lake Ontario from data on the abundance of each species and its eggs, and from predictions of egg development times. Bosmina longirostris is the most productive species at the nearshore stations whereas Diacyclops thomasi is the most productive offshore. Total crustacean zooplankton production is fairly uniform throughout the lake, both nearshore and offshore, with the possible exception of somewhat reduced production at the western end of the lake. Average total production was estimated at 15 g∙m−2∙yr−1. Using a recent estimate of particle-size-conversion efficiency in Lake Ontario, obtained from data on contaminant concentrations in aquatic biota, we predicted potential fish production in the lake for various sizes of fish. From published data on the commercial fish harvest from 1910 to 1930, a time period when commercial production was at a maximum, we estimate that the past commercial harvest for pelagic species may have been 20% or more of our predicted potential production.

nivel:~i~ qf Toronto, Toronto, Ont. MIC bA4 AND H. SHEAR Great L a k s Bialimieolr>gy Lobomtoql, Fisheries and 0ceara.r Cu~zadu, Burlir~ytora, Ont. L7R 446 NALEWAJKO, C . , K. LEE, AND H. SHEAR. 1981. Phosphorus kinetics in Lake Superior: light intensity and phosphate uptake in algae. Can. J. Fish. Aquat. Sci. 38: 224-232. Epilimnetic phytoplankton in Lake Superior in September, 1979, had low bk values (75-190 pE -mu" ss-'), low N/P ratios (8 to 13 : 1) and "~0 4 -P uptake kinetics that were not consistent with a state of extreme phosphorus limitation. Parallel laboratory experiments with Chlorells~ py~-elaoidosa indicated that phosphorus content per cell was higher and uptake rates of phosphate were lower in cells grown under low light (57 pE: . m-2 . s-') than those under high light (340 pE m-' . s-I). Maximum 3'~04 uptake occurred at about 50-60 pE -n~-~ . s-' in both cultures indicating 3 2~0 4uptake kinetics are light dependent at very low light levels, below or close to their bk values. It appears that light and not phosphorus limited phytoplankton growth in Lake Superior at the time of our experiments. Antecedent solar radiation prior to our experiments coupled with complete mixing of the top 20-25 m of the Lake possibly resulted in a low light-adapted phytoplankton population. We suggest that phosphorus control need not be the correct management strategy to maintain oligotrophy in Lake Superior.En septembre 1979, le phytoplancton Cpilimnetique dea lac Superieur avait de basses valeurs de Ik p8. E . n~-~ s-I), de fdibles rapports N/P (8 a 13 : 1) et une cinktique d'assimilation de "~0 4 -P en contradiction avec un Ctat d7extr2me limitation de phosphore. Bes expbriences paralltles en laboratoire avec CBzlorc.lla pyrenuiskPsta indiquent que le contenu en phosphore par cellule Ctait plus 6levC et les taux d'assimilation des hosphaaes moins klevCs dans les cellules cultivtes B une hible luminositk (57 pE * m-' . s-l) que dans celles cultivkes h une fane luminosit6 (340 pE --mu" ssl'). L'assimilation maximale de " '~0~ se produisit B environ 50-60 pE . m-' s-' dans les deux cultures, signe qeae la cinktique d'assimilation de 3 '~~4 dbpend de la lumitre a des trks faibles niveaux d'intensite Iumineuse, au-dessous ou se rapprochant de leur valeur de Ik. I1 semble que Ia lumikre, et non le phosphore, limitait la croissance d~a phytoplmcton dans le lac Supbrieur au anoment de nos expkriences. La radiation solaire avant nos experiences, couplke B un meIange conaplet des 20-25 m sup6rieurs du lac a probablement produit une population phytoplanctonique adaptCe i une faible intensite Iurnineuse. I1 est possible que Ie contr6le du phosphore ne soit pas nkcessairement la bonne stratkgie de gestion pour maintenir l'oligotrophie dms le lac Seapirieur .

No abstract

Lake Chapala is the largest natural water body in Mexico and also one of the most important shallow lakes in Latin America. For the past several years it has suffered various environmental problems such as the upstream overuse of water, contamination, and sedimentation. For the past 10 years the lake has had less than 50 percent of its historical water level over the past century. No criteria are reported in the literature that establish a water storage volume that will guarantee water quality conditions necessary for the survival of the lake. After determining the behavior of total solids concentrations in relation to the variations in the lake's depth, we proposed a minimum water column height of 5.0 m, representing a storage volume of about 5,000 Mm3. This volume would result in the recommended water quality standards for total dissolved solids. Calculated distribution maps show that the total solids concentration in the lake has increased since the end of the 1970s. The solids are primarily concentrated in the eastern part of the lake, as a consequence of the high solids discharged from the Lerma River, its main tributary.