Fatty acid (FA), total lipid, protein, amino acid, carbon, nitrogen, and phosphorus content was analyzed in 24 samples of freshwater microalgae. The samples originated from batch, continuous, or mass cultures in various growth phases and from net samples from lakewater. FA were analyzed quantitatively by using an internal standard in a GLC system and expressed as mg·g−1 dry weight (DW). The FA of one group of blue‐greens (e.g. Oscillatoria and Microcystis) were similar to those of the greens with higher amounts of 18C acids of the ω3 type compared to the ω6 type, whereas the other group (e.g. Anabaena and Spirulina) contained mostly ω6 acids. The flagellates, a taxonomically diverse group, were characterized by high amounts of long‐chained (20–22 C) polyunsaturated FA (PUFA), particularly of the ω3 type. The ω3/ω6 ratio appears to be highest in algae in the exponential growth phase. The increased lipid content in stressed algae was mostly due to increased saturated fatty acids and ω6 acids, whereas the valuable ω3 acids were unchanged or even decreased. Amino acid composition (% of total amino acids) did not vary much betaken species, but when analyzed quantitatively (mg‐g−1 DW), varied considerably between species and within species in different growth phases. The nitrogen and phosphorus contents were variable in all three algal groups. The relationship between PUFA and phosphorus content differed among the algal groups. The data suggest that PUFA in the phospholipids consist mostly ω3 acids.

The nutritional value of several planktonic algae was tested by means of feeding trials with three cladoceran zooplankters. The algae were monocultures and included two blue-greens, four greens and four flagellates with a size range of 5-48 u,m. The specific growth rates of the zooplankters were chosen as the measure of the nutritional value of the algae. The three cladocerans showed large differences in growth rate in the different algae, but the two cryptomonads were without doubt best suited as food for all. The fatty acid composition for the cryptomonads were different from the other algae. They contained high percentages of the polyunsaturated fatty acids 20:5u)3 (EPA) and 22:6w3 (DHA), which also are common in fish. It is suggested that the lipid composition is a probable factor determining the nutritional quality of the algae.

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In 56 samples of freshwater fish, most were low in fat, ≤ 5% of dry weight (D.W.), and the sum of all fatty acids (ΣFA) was about 2% of D.W. Trout, whitefish, and grayling had the highest content of the long‐chained FA. of ω3 type, EPA and DHA (1·7–2·6% of D.W.). Two large, low‐fat pikes with ΣFA of about12–3% of D.W. and a medium‐fat whitefish had the highest ω3/ω6 ratios, 8–9, whereas the fattiest fishes, eels from two lakes and the Baltic (ΣFA =17–26% of D.W.) had lower ω3/ω6 ratios, 1·1–1·8 (ω3 and ω6 FA are two important series of FA). The results indicate that ΣA controls the content of saturated FA (SAFA) and monounsaturated FA (MUFA), whereas the polyunsaturated FA (PUFA) was independent of ΣFA after a break point of about 10%ΣFA of D.W. The P/S ratio (PUFA/SAFA) and the PUFA/ΣFA ratio decreased with increased ΣFA, whereas the ω3/ω6 ratio showed no clear correlation to ΣFA. The difference in fatty acid patterns lay between low‐fat and high‐fat fishes, rather than between marine and freshwater fishes. The variation, both within and between species of the separate FA is small in fish with similar ΣFA content. Also, low‐fat and medium‐fat fishes tend to be more dietarily favourable than high‐fat fishes, when considering the latest criteria for high nutritional value to humans. Abbreviations used in the text: FA, fatty acids; ΣFA, sum of all FA; AA, arachidonic acid (20 : 4ω6); EPA, eicosapentaenoic acid (20 : 5ω3); DHA, docosahexaenoic acid (22 : 6ω3); SAFA, saturated fatty acids; MUFA, monounsaturated fatty acids; PUFA, polyunsaturated fatty acids; D.W., dry weight; F.W. fresh weight; CV, coefficient of variation; ω3 FA, series of PUFA with the first double bond located at carbon number 3; ω6 FA, series of PUFA with the first double bond located at carbon number 6. The fatty acids are described by three numbers, x:ywz, where x=number of carbon atoms, y=number of double bonds, and z=position of the first double bond counted from the methyl end of the molecule.

1. Plankton net samples and sedimenting matter in traps from mesotrophic Lake Erken, Sweden, were analysed for carbon (C), nitrogen (N), phosphorus (P), total lipids and fatty acid (FA) content to determine what differences and seasonal changes might exist in the quality of food available to pelagic zooplankters and benthic invertebrates. 2. Matter collected in a plankton net was assumed to correspond to food available to pelagic grazers, while matter collected in sedimentation traps provided a measure of food available to benthic invertebrates. Furthermore, food quality was assumed to be related to polyunsaturated fatty acid (PUFA) content. 3. The results suggest that suspended particulate matter, collected with a plankton net, is a much higher‐quality food resource than sedimenting matter. Our data also show that only during the spring and autumn do the benthic fauna have access to high‐quality food, because of the dominance of diatoms during these periods. During summer, pelagic grazer production was probably limited by food quantity, whereas benthic invertebrate production was probably limited by food quality. 4. Plankton net samples showed consistently higher values than trap samples (% of dry weight, DW, or mg g–1 DW) for nearly all variables analysed: C, N, P, saturated FA (SAFA), monounsaturated FA (MUFA), PUFA, ω3 FA and ω6 FA. If N and P were expressed per C, the differences decreased and both data sets indicated moderate to no N or P limitation. N/P also showed well‐balanced ratios throughout most of the season, on average 10 and 7 (by weight), respectively. The largest differences between the two food resources occurred in the PUFA content, including the important FA of the ω3 type. The only FA with higher levels in the trap samples were some unidentified FA of relatively short chain lengths. Seasonal variation for most variables was also very large in both net and trap samples. 5. PUFA and ω3 FA showed good relationships with the P content of net samples, but not with that of trap samples. Eicosapentaenoic acid alone did not give a good relationship with P. Thus, P seems to be a good predictor of food quality in living phytoplankton but not in dead matter or detritus.

Bacterioplankton abundance, [3H]thymidine incorporation, 14CO2 uptake in the dark, and fractionated primary production were measured on several occasions between June and August 1982 in eutrophic Lake Norrviken, Sweden. Bacterioplankton abundance and carbon biomass ranged from 0.5 x 109 to 2.4 x 109 cells liter-' and 7 to 47 ,ug of C liter-', respectively. The average bacterial cell volume was 0.185 ,um3. [3H]thymidine incorporation into cold-trichloroacetic acid-insoluble material ranged from 12 x 1012 to 200 x 10-12 mol liter-1 h-1. Bacterial carbon production rates were estimated to be 0.2 to 7.1 p.g of C liter-' h-1. Bacterial production estimates from [3H]thymidine incorporation and 14C02 uptake in the dark agreed when activity was high but diverged when activity was low and when blue-green algae (cyanobacteria) dominated the phytoplankton. Size fractionation indicated negligible uptake of [3H]thymidine in the >3-pum fraction during a chrysophycean bloom in early June. We found that >50% of the 3H activity was in the >3-p.m fraction in late August; this phenomenon was most likely due to Microcystis spp., their associated bacteria, or both. Over 60o of the 14CO2 uptake in the dark was attributed to algae on each sampling occasion. Algal exudate was an important carbon source for planktonic bacteria. Bacterial production was roughly 50% of primary production.

Temperature functions in biology and their application to algal growth constants Gunnel Ahlgren Ahlgren, G. 1987. Temperature functions in biology and their application to algal growth constants.-Oikos 49: 177-190. Various kinds of temperature rules have been proposed for biological use, but reasons for choosing one before another have seldom been given. Arguments for such choices should include both theoretical and mathematical-statistical aspects. In this paper the relationships of algal growth constants, such as ~t (maximum specific growth rate), q0 (minimum nutrient content of the algae), Y (yield coefficient) and Ks (halfsaturation constant for growth) with temperature (t) were investigated. The growth constants were estimated from growth experiments with the green alga Scenedesmus quadricauda performed in batch and P-limited chemostat cultures at t between 3 and 25°C. Additional growth data from different algal populations were estimated from 14C experiments in an incubator and in the field (0-20°C). The dependence of both (A and AZ (assimilation number) on t was generally described better by Belehradek's equation based on a "physical view", i.e., the rate of biological processes is more likely controlled by physical processes such as diffusion and viscosity than by equations of Berthelot's or Van't Hoff-Arrhenius' types, which were derived from chemical processes. Within smaller t intervals, Burckhardt-Harvey's equation (linear) often gave an equally good fit. For Scenedesmus the parameter q0 can be described by a 2nd degree polynomial. The limit value of Y at ,t = 0 versus t can also be best described by Belehradek's equation. Both Ks and Y at ut =-seem to be independent of t. Many examples from zoology also show statistically the most accurate fit to Belehradek's equation. On the whole, biological processes seldom show exponential increases with t. For example, the RGT-rule (Q0l), which is so widely used even today, often gives artificial "breaks" in the temperature coefficients. An equation of Belehradek's type should therefore be more generally accepted, also because its parameters appear to have some ecological significance.

Fatty acid (FA) concentrations and their seasonal variations were quantified for profundal benthic invertebrates, surficial sediment, and sedimenting matter from Lake Erken, Sweden. Food quality for profundal zoobenthos, as indicated by the concentrations of long-chain polyunsaturated FA, ω3 FA, or eicosapentaenoic acid (EPA) in sediment and sedimenting matter, was highest in spring and autumn and markedly lower in summer. Surficial sediment was consistently lower in all FA than was sedimenting matter. Palmitoleic acid (16:1ω7) was the dominating FA in both sedimenting matter and sediment. In fauna samples, EPA, palmitic acid (16:0), palmitoleic acid, and vaccenic acid (18:1ω7) were among the dominant FA. Docosahexaenoic acid was found only in the predators Chaoborus and Procladius. Differences between functional feeding guilds were found for the diatom-indicating FA palmitoleic acid and the bacteria-specific FA isoseptadecanoic acid (17:0iso). Furthermore, principal components analysis showed marked differences in FA composition among taxa. These differences reflect the relative contribution of food from autotrophic (phytoplankton production) and heterotrophic sources (detrital food web) in profundal invertebrate taxa.