(2000). Decomposition of the dinoflagellate Peridinium gatunense under oxic vs. anoxic conditions. SIL Proceedings, 1922-2010: Vol. 27, No. 2, pp. 879-883.

The decomposition of freeze‐dried whole cells and empty thecae of the dinoflagellate Peridinium gatunense Nygaard originating from dense blooms in Lake Kinneret (Israel) was followed experimentally under controlled conditions in the lab. The two materials (whole cells; empty thecae) were suspended in replicate bottles containing nutrient‐poor epilimnetic water from the lake. After 7 d, nutrients (N, P, and trace metals) were added to half the bottles. We followed the changes with time in dry weight, the dynamics of nutrients, microbial abundances and a range of microbial activities including leucine incorporation rates and activities of hydrolytic enzymes. Because of the low N and P content of thecae (C:N:P atomic ratios >3000:19:1) relative to protoplasts (276:51:1), the microbial utilization of thecae was expected to depend much more on the availability of external nutrient sources than the utilization of protoplasts. Indeed, decomposition of thecae did not occur in the absence of external nutrients but was rapid (1–2 d to their disappearance) after nutrients were added. In contrast, almost no stimulating effect of nutrient addition was observed for the decomposition of whole cells. The results suggest that intensive regenerative nutrient cycling or external nutrient inputs are a necessary precondition for an efficient trophic transfer of the energy stored in blooms of thecate dinoflagellates. The high nutrient demands of microbial degradation imply furthermore a competition for nutrients between heterotrophic degradative and phototrophic productive processes. Because of the generally assumed stronger competitive ability of heterotrophic bacteria, reduced primary production is expected as an indirect result of dinoflagellate bloom degradation. Indeed, reduced primary production is observed in Lake Kinneret every summer after the decline of the annual Peridinium bloom in June–July.

Microalgal biovolume is commonly calculated to assess the relative abundance (as biomass or carbon) of co-occurring algae varying in shape and/or size. However, a standardized set of equations for biovolume calculations from microscopically measured linear dimensions that includes the entire range of microalgal shapes is not available yet, In comparison with automated methods, the use of microscopical measurements allows high taxonomic resolution, up to the species level, and has fewer sources of error. We present a set of geometric shapes and mathematical equations for calculating biovolumes of >850 pelagic and benthic marine and freshwater microalgal genera, The equations are designed to minimize the effort of microscopic measurement. The similarities and differences between our proposal for standardization and previously published proposals are discussed and recommendations for quality standards given.

The filamentous cyanobacterium Aphanizomenon ovalisporum was observed for the first time in Lake Kinneret in August 1994 and formed a prominent bloom from September through October. Aphanizomenon ovalisporum reappeared in diminished amounts in the summer and fall of 1995. These events are the first record of significant quantities of a potentially toxic nitrogen-fixing cyanobacterium in this lake. No definite provenance of inoculum has been identified, although A.ovalisporum was also observed in a newly reflooded area (Lake Agmon) in the catchment. Unusually high water temperatures and low wind inputs were observed prior to and during the A.ovalisporum bloom period. These, together with possibly enhanced availability of phosphorus or other growth factors, may have contributed to the cyanobacterium growth in 1994. Phosphorus limi tation, as indicated by high cellular alkaline phosphatase activity, the onset of stormy conditions and a fall in water temperatures led to the demise of the 1994 bloom. Although the A. ovalisporum bloom in 1994 had no serious direct impact on water quality, the continued presence of a potentially toxic cyanobacterium in Lake Kinneret, a major national water supply source, is a cause for serious concern.

From the beginning of detailed routine monitoring in 1968 (and probably before) until the end of 1993, annual and seasonal development of phytoplankton in Lake Kinneret showed a relatively constant pattern. In the late winter and early spring there were extensive blooms of the dinoflagellate Peridinium gatunense, followed in summer and autumn by much lower standing stocks of predominantly nannoplanktonic chlorophytes. Although Microcystis spp. were prominent (usually concomitant with the dinoflagellates) in the early 1970s, subsequently this genus and other cyanobacteria were generally unimportant contributors to phytoplankton biomass. Since early 1994, however, a marked change in seasonal phytoplankton development has been noted. There were record high blooms of Peridinium in 1994 and 1995; in the following years, 1996 and 1997, no dinoflagellate bloom appeared. In the late summer and fall of 1994 an exceptional bloom of Aphanizomenon ovalisporum occurred; this was the first recorded instance of an outgrowth of a potentially toxic and N-2-fixing filamentous cyanobacterium in this lake. This organism has been observed in low numbers in each subsequent year. In 1995, 1996, and 1997 Microcystis also became prominent. Levels of primary production since 1994 have generally been higher than previous multi-annual averages.The quality of water in Lake Kinneret is to a large extent determined by the nature and activity of its phytoplankton community. Because of the lake's central role as a national supply source of good-quality water, the apparent breakdown of previously regular patterns of the annual and seasonal algal development is a cause for concern. Integrated research and monitoring are needed to fully explain the causes of the observed and ongoing changes and to provide guidelines for future management policies aimed at maintaining water quality in Lake Kinneret.

Temporal changes in the abundance of Peridinium gatunense Nygaard in the water column of warm monomictic Lake Kinneret were followed during 1990–1994. Sedimentation rates of this dinoflagellate were followed concurrently by means of sediment traps with and without a preservative (Formalin), positioned at the base of the epilimnion and within the hypolimnion, for exposure periods of 2–3 weeks. Upper trap catches of total P. gatunense (live cells + dead cells + thecae + protoplasts + cysts) were nearly always higher than lower trap catches, partly due to decomposition of the cells as they sank through the water column. Over the S‐year period, total P. gatunense sedimentation rates ranged over 4 orders of magnitude, from values <0.001 to 8.5 g (WW) m−2 d−1. A typical seasonal pattern was observed in which sedimentation rates were relatively low during the bloom increase phase, with thecae (from cell division) being the main component, and increased substantially after the peak of the bloom, when the relative contribution of senescent cells, dead cells and protoplasts increased substantially. Cysts were trapped in low numbers, usually 1–2 orders of magnitude fewer than live cells. Interannual variations in total P. gafunense sedimentation were large and independent of the size of bloom—the proportion of annual P. gatunense production reaching the hypolimnetic traps ranged from 6% in 1994, the year with the largest bloom, to 68% in 1991, a year with an average‐size bloom. The high value was exceptional and we speculated that it resulted from higher resuspension and more severe nutrient limitation of microbial decomposition during that low water level, drought year. On average, thecae accounted for 75% of total P. gafunense sedimentation despite being only 55% of the P. gatunense‐produced biomass, suggesting that thecae were more refractory or less grazed than protoplasts. Thecal C:N:P ratio of <3,000:19:1 (vs. 276:51:1 for protoplasts) indicated that microbial decomposition of thecae is likely to require N and P inputs from other sources. Ultimately, our study highlights for the first time that annual dinoflagellate sedimentation rates may vary dramatically as a result of other processes such as decomposition, resuspension, and grazing, leading to dramatic variations in the amount of organic matter reaching the bottom sediments.

Lake Agmon, a small shallow water body (area 1.1 km(2), mean depth <1m) was created in April 1994 as part of the Hula restoration project in the dried pear-soils of the Hula Valley. Until ca. 50 years ago, this area was covered with swamps, extending to the north of Lake Hula. We followed changes over time in the abundance and species composition of the algal populations in Lake Agmon over the initial 4 years that followed its creation, consolidated the existing information on the algal populations of the extinct Lake Hula, and compared the Lake Agmon algal populations with those reported from Lake Hula and with those present in Lake Kinneret. Altogether, 276 algal species were found in Lake Agmon, including 140 chlorophytes, 48 euglenophytes, 34 cyanophytes, 31 diatoms, 8 cryptophytes, 8 dinoflagellates, and 4 chrysophytes. A comprehensive species list for Lake Hula was also compiled, based on the limited published accounts. The similarities between the past and present algal communities in the Hula Valley were great: most diatom, dinoflagellate, chrysophyte, euglenophyte, and large chlorophyte and cyanophyte genera that are seen today in Lake Agmon were also reported from Lake Hula. However, the Hula list of genera was shorter than the Lake Agmon list in some particular categories. The lack of most of the nannoplanktonic Chlorococcales, and all cryptophytes and other small flagellates from the Hula list was attributed to different sampling and preservation methods in the early days; the absence of most filamentous cyanobacteria is considered a real difference, possibly resulting from the more eutrophic status of Lake Agmon. Notably, the dinoflagellate Peridinium gatunense, which blooms annually in Lake Kinneret downstream of the Hula Valley, was not recorded in Lake Hula and did not occur in Lake Agmon.

The dynamics of the algal populations of Lake Agmon, a newly created shallow lake in the Hula Valley, Israel, were monitored following its filling in April 1994 through 1996. Additional limited field observations and measurements were taken throughout 1997. Following an initial establishment period, the dynamics of the algal populations showed a repetitive annual pattern comprised of three phases: I. a clear water phase in January–February, with low phytoplankton biomass and no metaphyton; II. a metaphyton dominance phase during March–June when mats of filamentous chlorophytes covered most of the lake's sediments while phytoplankton biomass remained low; and III. an intense phytoplankton bloom phase from June till December. The shifts from phase I to II and from phase II to III were gradual, resulting from interplay between phosphorus availability, the underwater light climate, temperature effects and zooplankton grazing pressure. The shift from phase III back to phase I was abrupt, due to winter flushing of Lake Agmon. The summer phytoplankton blooms intensified from 1994 to 1996 and shifted from chlorophyte dominance in 1994 and 1995 to cyanobacteria-dominance in 1996 and 1997. These observations, jointly with the nutrient chemistry of Lake Agmon, suggest intense eutrophication. Criteria based on phytoplankton taxonomy also indicate that Lake Agmon is eutrophic to hypertrophic. Due to the typical unstable nature of hypertrophic systems, careful management is essential to maintain the delicate ecological balance needed to ensure that the lake will fulfill its intended role as a center for eco-tourism.

Photosynthetic pigments extracted from the particulate material of the water column of Lake Kinneret were studied throughout the periods of May 1988-June 1989, and November 1993-November 1994, by means of HPLC The temporal and vertical variation of the pigment suite found agreed with the microscopically determined phytoplankton record. The regression calculations of taxon-specific biomass with the corresponding signature pigments suggest that pigment analysis may be a useful tool for the monitoring of bloom-forming species, e.g. the dinoflagellate Peridinium gatunense Nygaard. The HPLC pigment analysis permitted the identification and quantification of chlorophyll degradation products, providing for the first time information about their composition in Lake Kinneret. Chlorophyllide a was the major detectable degradation product of chlorophyll a, varying between 1 and 9% of the chlorophyll a concentration. Other chlorophyll a derivatives appeared mostly in minor quantities. Pheophytin a was virtually lacking in all the samples Removal rates of pigments measured by sedimentation traps, indicated that the degradation of chlorophyll a via chlorophyllide a is a dynamic process that continues during the sedimentation of the phytoplankton particles.

The response of natural phytoplankton populations from Lake Kinneret to the addition of iron (Fe) and chelator (EDTA) was tested by following growth patterns and determining the algal composition initially and at the end of the experiments; algal growth rates and yield were measured by in vivo chlorophyll fluorescence. Although the pattern of growth response varied, usually some stimulation of phytoplankton growth rate and yield was observed with chelator and/or Fe addition in comparison to unsupplemented samples. However, the major impact of Fe and EDTA on the phytoplankton appeared to be expressed as changes in the algal population composition. There were clear taxonomic differences in the response to Fe addition, which in nearly all experiments stimulated outgrowth of Bacillariophyta and Chrorophyta, compared with the effect of EDTA alone or with Fe, which enhanced the development of Cyanophyta, in addition to B acillariophyta. The availability of Fe and/or chelators therefore appears to be important in determining the composition of the phytoplankton populations in Lake Kinneret and presumably in other aquatic environments.

The long‐term (22 yr) record of primary production in Lake Kinneret, Israel, has been examined, together with chlorophyll and microscopically determined algal biomass, in order to discern whether there have been any significant changes in these parameters during this period. During the period 1972 through 1993, annual averages have ranged from 1,223 to 2,311 mg C m−2 d−1 for primary production, from 127 to 246 mg m−2 for chlorophyll, and from 39.3 to 98.5 g m−2 for algal wet weight. The annual peak of these parameters was in April–May. Over 22 yr, variability in primary productivity was more closely related to changes in chlorophyll than to changes in algal biomass. No evidence was found for consistently increasing long‐term trends in primary production, chlorophyll concentrations, or algal wet weight biomass from 1972–1993. Although the annual and semiannual averages of algal biomass were significantly higher in the past 11 yr than those in the previous decade, this pattern could arise from a long‐term cyclical but self‐compensating trend. The extended record indicates that despite population growth and intense economic development around the lake and in its catchment area, there has been no extreme eutrophication of Lake Kinneret from 1972 to 1993. We suggest that this relative resiliency of the Kinneret ecosystem is due to high ambient levels of alkalinity, calcium, and pH in the lake water acting to limit phosphorus availability, which in turn restricts the outgrowth of phytoplankton.

The dynamics of theCeratium hirundinella population and the abundance of dinocysts in the plankton and sediments were studied in Lake Sempach in 1988. In 1987, a rich population ofCeratium (380 cells ml−1) accompanied byPeridinium spp. developed in the lake. The dinocysts were found entrapped in a kind of flocs, in the deepest part of the lake, in the upper flocculent layer. The number of viable cysts ofCeratium in the sediments decreased gradually from April to July 1988. TheCeratium population increased slowly starting in April, and reached a maximum number in August (31 cells ml−1).Peridinium willei reached 100 cells ml−1. Newly formed cysts ofCeratium were recorded in the plankton and sediments at the end of July — beginning of August. They appear in the sediments as separate cells. Their number increased gradually, reaching a maximum of 600 cysts l−1 at the end of October.Ceratium formed more cysts than didPeridinium, but the rate of survival of theCeratium cysts appears to be lower than that ofPeridinium cysts. In addition to their biological functions, the cysts also have an impact on the ecosystem as carriers of nutrients from down to up and from up to down.

The general features of phytoplankton seasonal succession, abundance and distribution in Lake Kinneret, as based on observations from 1970 through 1989, are summarised. Throughout this period of observation, the large, thecate dinoflagellatePeridinium gatunense formed an annual, late winter to early spring bloom resulting in very high standing stock levels. The dominance of these dinoflagellates has a profound impact upon the lake ecosystem. In the summer and fall the phytoplankton assemblage consisted mainly of nanoplanktonic green and blue-green algae and diatoms. Picophytoplankton, mainly picocyanobacteria, were present in low numbers during the dinoflagellate bloom but reached maximum abundance (105 cells · ml−1) in the epilimnion during the summer and fall. Within a given year, chlorophyll concentrations correlated well with estimates of wet weight biomass, derived from microscope counts. However, interannual averages of chlorophyll did not correlate closely with those for wet weight biomass. Both wet weight biomass and chlorophyll standing stocks fluctuated more than 2 fold from 1970 through 1989 but no extreme, long-term, continuous trend of increase or decrease was observed. Thus, phytoplankton has remained relatively stable although there has been a significant rise in the levels of summer-fall biomass since 1981. The main factor responsible for this may have been increased available phosphorus; the abundance of phytoplankton did not show any clear, long-term relation to that of herbivorous zooplankton.

The phytoplankton of Lake Kinneret (Israel), a subtropical lake, is characterized by a yearly winter-spring water bloom of dinoflagellates. The identity, abundance, succession and temporal and spatial distribution of the dinoflagellate associations are presented. The common species are Peridinium gatunense, Peridiniopsis borgei, Ps. elpatiewskyi, Ps. cunningtonii and Ceratium hirundinella. The species present in the lake are forms with a wide range of geographical distribution. The development of the vegetative cells and their bloom occur in similar environmental conditions in Lake Kinneret and in temperate water bodies. The unfavourable conditions for the vegetative cells are in summer in the subtropical lake and in winter in temperate zones. Thus, the cysts of P. gatunense, Peridiniopsis spp. and C. hirundinella are "oversummering" forms in Lake Kinneret and "overwintering" ones in temperate water bodies.

A long-term study (1969-1989) of the phytoplankton of Lake Kinneret revealed that Chroococcus minutus has been the most important contributor to the Cyanophyta biomass. During this period, large fluctuations in the abundance of the Chroococcus population occurred. The abundance is correlated with the increase or decrease of nutrients, with the decrease of grazing pressure by herbivorous zooplankton, and with the intensity of the dinoflagellate water bloom, due to shading.