Limnological and ecophysiological aspects of Aphanizomenon ovalisporum bloom in Lake Kinneret, Israel

The first appearance of Aphanizomenon ovalisporum in Lake Kinneret in August 1994 was apparently boosted by relatively high concentrations of total dissolved phosphorus (12 g P l^-1 as compared to an average of 8 g P l^-1). The increasing Aphanizomenon biomass in a lake in which phytoplankton are generally phosphate limited in summer and autumn was accompanied by high enzymatic activity of alkaline phosphatase, reaching values of 2830 nmol MU l^-1 h^-1, suggesting a great demand for phosphorus. In addition, the nitrogen requirement of the developing population of Aphanizomenon was partly provided by nitrogen fixation, as indicated by a high percentage of heterocysts. Laboratory experiments demonstrated that filtrate from an old Peridinium gatunense culture enhanced Aphanizomenon growth. Thus, it is postulated that the degradation of the massive Peridinium bloom in spring and early summer supported the development of A.ovalisporum. The high pH and alkalinity during the bloom of Aphanizomenon indicate that A.ovalisporum is probably a HCO3^- user. After 1994, akinetes of A.ovalisporum were left in sediments and the water column, and could be a source for the next year's bloom. This possibility was demonstrated by inoculation of lake water and sediments into nitrogen-depleted BG-11 medium, resulting in the dominance of A.ovalisporum.      

Physiological variables determined under laboratory conditions may explain the bloom of Aphanizomenon ovalisporum in Lake Kinneret

Response of Aphanizomenon ovalisporum to certain environmental parameters was studied to gain a better understanding of the conditions which may have stimulated its autumnal bloom in Lake Kinneret. Optimal temperature for A. ovalisporum growth was 26–30?°C, resulting in growth rates of 0.2–0.3?day^?1, similar to those observed in the lake. Maximal rate of CO₂ fixation (assimilation numbers of 6–8?μg?C?μg^?1?Chl?h^?1) was obtained at low irradiances (I _k of 40–100?μmol?photons?m^?2?s^?1), 200?μM Pi and low N:Pi ratios. Growth was strongly affected by phosphorus availability, reaching a maximum at Pi concentrations above 40?μM. The high demand for phosphorus was indicated by an increase in alkaline phosphatase activity. The relative abundance of Pi in the cells increased by 4-fold in Pi-rich compared with Pi-limited cultures. Uptake of Pi was faster in Pi-depleted compared with Pi-sufficient cells. Maximal photosynthetic rates and K_1/2(HCO₃ ^?) were 140–220?μmol?O₂?mg^?1?Chl?h^?1 and 10–24?μM, respectively. At pH 7.0 the K _1/2(CO₂) was 2.2 and fell to 0.04?μM at pH 9.0. These data indicated that A. ovalisporum is a HCO₃ ^? user, and can explain its high photosynthetic rates during the bloom, under high pH and low dissolved CO₂ conditions. Na⁺ concentrations of about 5?mM were essential for A. ovalisporum growth at high pH approaching values in the lake.     

Annual changes in the mixed depth and critical depth for photosynthesis by Aphanizomenon ovalisporum that allow growth of the cyanobacterium in Lake Kinneret, Israel

A limnological database for Lake Kinneret was used to determinethe period each year when the interaction of mixed depth andlight availability would permit growth of the planktonic cyanobacteriumAphanizomenon ovalisporum. The organism, recorded for 56–211days in each of the 7-years, 1994–2000, was usually distributedthrough the mixed epilimnion and reached a maximum in late October.The mixed depth (z_m) was calculated from the temperature profile,wind speed and wind direction. Changes in the total populationwere related to changes in the daily integrals of photosynthesis,calculated from vertical profiles of water temperature () andlight attenuation (K_d), and hourly changes in photon irradiance(E₀) each day, using temperature-compensated algorithms basedon measurements of photosynthesis at different irradiances.Net photosynthesis was calculated for different ratios of respiration(R) and maximum photosynthesis (P_m). In a modelling analysis,the daily integral of net photosynthesis fell below zero onthe day of the population maximum, and remained negative duringthe period of population decline, when R was set at -0.08 P_m.Similar modelling, based on the 7-year mean values of , K_d andE₀for each day of the year, showed the critical depth for photosynthesisby Aphanizomenon would exceed the mixed depth in Lake Kinneretfrom mid-March to late October. Direct measurements of -R/P_mratios could be used with information in the database to analysethe periods of photosynthetic production by different phytoplanktonspecies in Lake Kinneret.     

First report of the cyanobacterium Aphanizomenon ovalisporum Forti in two Greek lakes and cyanotoxin occurrence

Aphanizomenon ovalisporum is reported for the first time inGreece, in two warm, monomictic lakes. Aphanizomenon ovalisporumwas dominant constituting 99 and 58% of the total cyanobacterialbiomass in lakes Lysimachia and Trichonis, respectively. Trichomeswere solitary (length 60–700 µm), were narrowedslightly at the ends, had a few terminal hyaline cells and hadcells containing gas vesicles (length 2.5–6.9, width 2.4–5.1µm). Heterocytes, spherical or ellipsoidal (length 4.4–10.5,width 2.41–5.1 µm) and akinetes (length 16.0–27.8,width 6.0–15.9 µm) were located in the middle ofthe trichome. High performance liquid chromatography (HPLC)analysis detected microcystin–LR (MC–LR) and a putativeanabaenopeptin in the L. Lysimachia sample. The sestonic MC–LRconcentration was 0.9 µg L^–1. The origin of MC–LRin L. Lysimachia is discussed. The other cyanobacteria presentwere Pseudanabaena sp. and Planktothrix mougeotii (1% of thetotal cyanobacterial biomass).     

Decreased salinity effects in Lake Kinneret (Israel)

Kinneret is the only freshwater lake in Israel. It currently supplies about 30% of national water demands. Most pumped water is for drinking, and water quality is of major concern. During 1970–1987 temporal changes were observed in the lake ecosystem: decrease of salinity, decrease of total N (TN) and increase of total P (TP) mass contents, decline of TN/TP ratio, increase of phytoplankton biomass and increase of algal photosynthetic specific activity. It is suggested that because of decrease in salinity carbonic anhydrase activities in algal cells and nitrifying bacteria were enhanced. The increase of NO₃ flux through nitrification consequently enhanced denitrification and nitrogen losses in lake waters. These increased N losses together with P increase, as reflected by the decline of TN/TP ratio might be a slight shift from the present both P and N deficiencies to a higher level of N limitation in the Kinneret ecosystem. This This may cause changes in phytoplankton community structure possibly without changing primary production levels but deteriorate water quality.     

Phytoplankton periodicity in a subtropical lake (Lake Kinneret,Israel)

Lake kinneret is a subtropical monomictic lake characterized by a Pyrrhophyta-Chlorophyta assemblage, supplemented by Cyanophyta in some years. Concerning their abundance and seasonal occurrence, the phytoplanktonic algae belong to two groups: algae appearing in quantity at a definite annual period and algae present throughout the year. Four stages of algal succession occur in the lake. There is a marked periodicity in the phytoplankton composition with a high standing stock in winter-spring, due to the dinoflagellate water-bloom, and a low one during the summer months, related to the high stability of summer stratification. The annual succession at the species level has been an almost constant event in the lake for many years.The increase in nutrient concentrations in 1973 and 1974 increased the diversity and abundance of algae (except Peridinium) but did not lead to significant changes in algal succession. Conversely, the decrease of the zooplankton grazing pressure in 1975 and 1976 facilitated the development of algal maxima during summer-fall. They were caused by nanoplanktonic forms, and they developed without additional enrichment of nutrients. The algal abundance and diversity decreased. The years 1981 and 1982 were characterized by both an increase in phosphorus and a decrease in zooplankton. These conditions favored the concomitant abundance of many species and an increase of non-Pyrrhophyta biomass.     

Metalimnic Layer in Lake Kinneret,Israel

Summary Chemical and biological investigations of Lake Kinneret showed that reducing conditions prevailling in summer in the metalimnion cause an early and intense denitrification process accompanied by the release of dissolved Fe and Mn. These conditions are favourable to the development of sulfo-bacteria and blue green algae. The role of this layer is discussed.     

The productivity of mesocyclops leuckrarti (Claus) in Lake Kinneret (Israel)

Specific rates of growth (Cw) of Mesocyclops leuckarti, young male and female instars varied between 0.03–0.26 and 0.03–0.17 g_(w.w.)/g_(w.w.)/day respectively at 15° and 22°C, whilst at IV–V copepodid stages females showed a higher Cw values. During 1969–1975 the averages of productivity and monthly P/B ratios were 44 (±23) g_(w.w.) (= 5g_(w.w.)/m²/month) and 3.1 respectively. P/B ratios were highly correlated (r(sup2)=0.98) with temperature changes. Metabolic parameters and P/B ratios were found to be similar to other water bodies in the world indicating an adaptation of M. leuckarti to different conditions.     

Sulfte-reduction process in sediments of Lake Kinneret,Israel

Monomictic Lake Kinneret is stratified during summer and autumn, resulting in a hypolimnion rich in H₂S (3–7 mg 1^–1). In winter and spring every year a bloom of dinoflagallate Peridinium gatunense produces an average biomass of 150000 ton wet weight. Part of this biomass sinks to the hypolimnion and sediments where it is decomposed and mineralized, with some of the mineralization due to the activity of sulfate-reducing bacteria (SRB). The sulfate-reduction potential of the upper sediment layer at the deepest part of the lake (42 m) was measured. The activity of the enzyme arylsulfatase was also monitored. Rates of sulfate-reduction ranged from a minimum of 12 nmoles SO_f4 ^p2–-reduced cm^–3 day^–1 in December before lake overturn to a maximum of 1673 nmoles SO_f4 ^p2– reduced cm^–3 day^–1 in July during stratification. These rates are considerably higher than those recorded from other freshwater lakes in the world and are probably limited more by the availability of organic matter than by sulfate concentrations.     

Seasonal dynamics of picophytoplankton in Lake Kinneret, Israel

1. Picophytoplankton (picocyanobacteria and picoeukaryotes) communities in Lake Kinneret were studied from 1988 to 1992. No prochlorophytes were observed in the lake. 2. Picocyanobacteria were a prominent and ubiquitous component of the phytoplankton, being present at all depths throughout the year, with concentrations ranging from 2 ± 10–8 ± 10⁵ cells ml^?1. Low cell numbers in winter and spring were followed at the end of the annual dinoflagellate bloom by maximal abundances in summer-autumn in the epilimnion. High cell numbers (> 10⁴ cells ml^?1) were sometimes also found in the anaerobic hypolimnion. Net growth rates for picocyanobacteria ranged from 0.29 to 0.60 divisions day^?1. 3. Picoeukaryotes were a very minor constituent of the picoplankton, mostly present in winter and spring, and sometimes at the end of autumn, with concentrations ranging from 44 to 5700 cells ml^?1. Higher cell numbers tended to occur in the near surface water layers. In August-September, picoeukaryotes were found only in the hypolimnion. In December, the occurrence of picoeukaryotes in the deep water layers probably resulted from advection with cold water currents from the Jordan river. Net growth rates for picoeukaryotes ranged from 0.26 to 0.43 divisions day^?1. 4. Overall, the contribution of picophytoplankton to the phytoplankton standing crop in Lake Kinneret was limited; picocyanobacteria and picoeukaryotes accounted for no more than 7.0 and 0.1% of total algal biomass (semiannual average), respectively. 5. Picophytoplankton cell numbers in pelagic waters were usually similar to those in shallower lake stations. 6. Picocyanobacteria appear to be an autochthonous population, whereas picoeukaryotes are probably brought annually by the Jordan River and do not maintain themselves in the lake.     

Sex ratio in Mesocyclops leuckarti (Claus) populations in Lake Kinneret (Israel)

Sex ratio (F/M) values of adult Mesocyclops leuckarti (Claus) populations in Lake Kinneret during 1970 and 1974 showed a periodic pattern of winter (January–March) and summer (August–October) female peaks. It is suggested that water temperature governs the appearance of both peaks but through different pathways.Contribution no. 163, Israel Oceanographic & Limnological Research Ltd.     

Alternative states in the phytoplankton of Lake Kinneret, Israel (Sea of Galilee)

1. Through analyses of a 34‐year record of phytoplankton, zooplankton and physicochemical parameters from Lake Kinneret, Israel, we show that distinct and persistent phytoplankton assemblage states occurred from winter to summer. 2. The most obvious characteristic of these states was the presence or absence of a spring bloom of the dinoflagellate, Peridinium gatunense. 3. Analyses of the data within the framework of the alternative states model revealed a possible complex triggering mechanism, and system hysteresis. 4. A change in zooplankton biomass and body size coincident with changes in predation pressure associated with the collapse of the Kinneret Bleak, Acanthobrama terraesanctae, fishery appeared to be the ‘slow changing’ variable in the context of the alternative states model. Alternative phytoplankton states were only possible after this variable crossed a threshold in 1993–94, following the collapse of the fishery. 5. When alternative states were possible, some physicochemical parameters and the structure of the zooplankton assemblage appeared to control which phytoplankton state emerged in a given year. In years without a P. gatunense bloom, important physicochemical parameters in winter included low NO₃ loading, high water temperature, high water level, a deeper thermocline, low transparency, high concentrations of NO₃ and Cl in the epilimnion, and low concentration of epilimnetic total phosphorus. In addition, the cladoceran Chydorus sphaericus and adults of the copepod Mesocyclops ogunnus were observed in winter in years without a bloom. 6. Zooplankton biomass and body size of some taxa have recovered since the 1993–94 collapse of the fishery, yet incidence of both phytoplankton states in Lake Kinneret was still possible. Within the framework of the alternative states model, this suggests that the slow changing variable threshold where alternative states became possible is different from the threshold where alternative states will no longer be possible. In other words, the system is characterised by a hysteresis.     

Dissolved organic nitrogen utilization by an Aphanizomenon bloom in Lake Kinneret

The concentrations of dissolved organic nitrogen (DON) and dissolvedinorganic nitrogen (DIN) in the epilimnetic waters of Lake Kinneretdecreased from 371 to 152 µg N 1^–1 and from 65 to13 µg N 1^–1, respectively, during September-October1994, concomitantly with the outgrowth of the cyanobacteriumAphanizomenon ovalisporum. A mass balance estimate based onthe cyanobacterial standing stock indicates that the maximum,net N requirement of A.ovalisporum was between 106 and 245 µgN 1^–1 These observations suggest that components of theDON pool were a major direct or indirect source of N for themicroplankton and that N fixation was not a significant factorin the development of A.ovalisporum. Further support for thisidea was given by experiments which showed that monoculturesof this organism grew well in mineral media supplemented withorganic N com pounds (urea, hypoxanthine, lysine, guanine, glucosamine).In general, it is proposed that the DON pool, which is oftenin relatively high concentrations compared to DIN, must be consideredan active participant in N recycling and may serve as an importantN source for phytoplankton in many aquatic environments.     

Food and feeding habits of Mirogrex fingerlings in Lake Kinneret (Israel)

The food composition of the fingerlings (13–41 mm TL) of Mirogrex terraesanctae terraesanctae was studied on specimen collected at four stations in Lake Kinneret. The fingerling feed mostly on pelagic zooplankters and zoobenthic forms whereas algae were found in the intestines in low biomass. The number of zooplankton organisms in the guts of the fingerling that were sampled during June–August was higher by 234% than in fingerlings that were sampled during March–April. The role of sardines fry in the Kinneret ecosystem is discussed.     

Non-siliceous algae in a five meter core from Lake Kinneret (Israel)

The composition and succession of non-siliceous algae, studied in a five meter core from Lake Kinneret (Israel), are described. Only Chlorophyta species were recorded, probably due to the standard palynological sample processing which was used. In the lower part of the core, from the bottom to 300 cm (interval 5500–2500 years B.P.), Botryococcus braunii was the only common alga. Relevant changes in algal diversity and abundance occur at 300 cm. Many species of green algae were recorded for the first time (Pediastrum, Scenedesmus, Coelastrum, etc.). These changes may be related to an increase in nutrient concentration as a consequence of cultural disturbance. In the interval 300-0 cm, a succession of Pediastrum species is followed. The recovered green algae are extant in the present plankton of Lake Kinneret. They also constitute an important part of the algae found in the profundal sediments today.     

Eukaryotic picoplankton, Mychonastes homosphaera (Chlorophyceae, Chlorophyta), in Lake Kinneret, Israel

Eukaryotic picoplankton was isolated from Lake Kinneret, Israel. This green picoplankton is spheroidal or ovoidal in shape. The cell wall has a network of ribs on its surface. The chloroplast is cup- or mantel-shaped without a pyrenoid. The morphology and cellular structure of this autosporic alga is identical with Mychonastes homosphaera (Skuja) Kalina et Pun?ochá?ová. The phylogenetic tree based on 18S ribo-somal RNA gene sequence data showed that M. homosphaera is placed in the Chlorophyceae and located at the base of a branch leading to the other algae possessing directly opposed orientation of the flagellar apparatus.     

Appearance and establishment of diazotrophic cyanobacteria in Lake Kinneret,Israel

1. We propose that the appearance and establishment of Nostocales (cyanobacteria) species of the genera Aphanizomenon and Cylindrospermopsis in the warm subtropical Lake Kinneret (Sea of Galilee, Israel) from 1994 was linked to changes in climate conditions and summer nitrogen (N) availability. 2. From 1994 to 2009, an increase in frequency of events of elevated water temperature (>29 °C) in summer, and to some extent a greater frequency of lower summer wind speed events, affected water turbulence and water column stratification, thus providing better physical conditions for the establishment of these populations. 3. In recent years, N‐depleted conditions in Lake Kinneret in early summer have promoted the development and domination of Nostocales that could gain an ecological advantage owing to their N₂‐fixing capability. 4. Nitrogen fixation rates coincided both with heterocyst abundance and with Nostocales biomass. The N supplied to the lake via nitrogen fixation ranged from negligible quantities when Nostocales represented only a minor component of the phytoplankton community to 123 tonnes when Cylindrospermopsis bloomed in 2005. This high N₂ fixation rate equals the average summer dissolved inorganic nitrogen load to the lake via the Jordan River.     

Seasonal variation in pigmentation of the dinoflagellate Peridinium gatunense (Dinophyceae) in Lake Kinneret, Israel

1. Pigment composition was measured in natural phytoplankton samples from Lake Kinneret, Israel. From March through June 1998, the dinoflagellate Peridinium gatunense Nygaard mostly contributed more than 95% of the algal biomass. Peak densities were found in April, close to the water surface, with >10⁹ cells m^?3, chlorophyll (Chl) a concentration of 380 mg m^?3 and areal Chl‐a density of >1300 mg m^?2. 2. Cellular concentrations of Chl‐a changed between 201 and 282 pg cell^?1, but did not show a defined temporal fluctuation. 3. The mass ratio of Chl‐c to Chl‐a changed from March to June between 0.16 and 0.22, and the peridinin to Chl‐a ratio changed from 0.25 to 0.41. Neither ratio showed a clear pattern of seasonal change. Conversely, there was a progressive increase in diadinoxanthin and β‐carotene ratios to Chl‐a through the season, parallel to the increase in photon flux impinging upon the lake surface. The diadinoxanthin to Chl‐a ratio changed from 0.11 to 0.28 and the β‐carotene to Chl‐a ratio varied from 0.03 to 0.08 from March through June. 4. Diatoxanthin was not detected in natural samples. However, it was present in experiments with P. gatunense cultures, when concentration of diatoxanthin increased rapidly, concurrent with a decrease in diadinoxanthin and β‐carotene concentrations, while Chl‐c and peridinin ratios to Chl‐a were almost stable with photon flux increase. 5. The seasonal variation in cellular pigmentation of P. gatunense in Lake Kinneret suggests that accumulation of photoprotective pigments is essential for optimisation of photosynthetic activity of this large dinoflagellate.     

The distribution of chlorophylls, carotenoids and their breakdown products in Lake Kinneret (Israel) sediments

SUMMARY. 1. The sedimentary distribution of chlorophylls, carotenoids and their breakdown products following the winter-spring bloom of the dinoflagellate Peridinium gatunense in Lake Kinneret was determined both spatially and vertically within the sediments, using high-pressure liquid chromatography (HPLC).
2. All four stations (epilimnic, oxic St. J (depth = 8m) and D (12m) and hypolimnic, anoxic St. F (21 m) A (41 m) were characterized by the presence of chlorophyll a (chl-a) chl- b , chl- c , phaeophytins a (ph'tin- a ) and b , phaeophorbide a (ph'bide- a ) and lutein, fucoxanthin, diatoxanthin and β-carotene together with spectrally similar, but HPLC-resolved, breakdown products in the surface sediments. The central deep station A (41 m), which is seasonally anoxic, trapped and preserved significantly more pigments (e.g. chl- a = 16.35 μg g⁻¹ organic matter) than the more erosive, oxic littoral stations (chl- a = 1.10 and 0.76μg g^−l OM at St. D and J, respectively).
3. Whereas it was possible to ascribe the presence of lutein and chl- b to inputs of green algae and higher plants from the catchment, and fucoxanthin and chl- c ₁, c ₂ to winter diatom blooms (e.g. Melosira granulata ), there was a remarkable absence of peridinin and dinoxanthin from sediments, despite the dominance of dinoflagellates in the phytobiomass. This is consistent with the well-known oxic lability of this epoxy carotenoid in aquatic systems, and indicates efficient breakdown of this dinoflagellate organic matter in the water column, prior to sedimentation.
4. Although pigment concentrations all decreased with depths within sediments, lutein and β-carotene were selectively preserved compared to chl-a and fucoxanthin which, at 40m depth at St. A, decreased to < 1% of their surface concentrations.     

Seasonal photosynthetic activity of autotrophic picoplankton in Lake Kinneret, Israel

Autotrophic picoplankton populations in Lake Kinneret are composedof picocyanobacteria and picoeukaryotes. Overall, the ratesof photosynthetic carbon fixed by autotrophic picoplankton duringthis study were low (0.01–1.5 mg Cm^–3 h^–1).The highest chlorophyll photosynthetic activity of the <3µm cell-size fraction was found in spring, when picoeukaryotespredominated and in addition small nanoplankton passed throughthe filters. The maximum cell-specific photosynthetic rate ofcarbon fixation by picocyanobacteria and picoeukaryotes was2.5 and 63 fg C cell^–1 h^–1, respectively. The highestspecific carbon fixation rate of autotrophic picoplankton was11 µg C µg^–1 Chl h^–1 The proportionalcontribution of autotrophic picoplankton to total photosynthesisusually increased with depth. Picocyanobacteria collected fromthe dark, anaerobic hypolimnion were viable and capable of activephotosynthesis when incubated at water depths within the euphoticzone. Maximum rates of photosynthesis (P_max) for picocyanobacteriaranged from 5.4 to 31.4 fg C cell^–1 h^–1 with thehighest values in hypolimnetic samples exposed to irradiance.Photosynthetic efficiency (