Jens Petter Nilssen

Tropical lakes — functional ecology and future development: The need for a process-orientated approach

The major classes of tropical lakes include shallow, lowland lakes; deep, tertiary lakes; high altitudinal lakes; rainforests lakes; and man-made lakes at all latitudes and altitudes. Basic ecological processes are similar in temperate and tropical lakes, including grazing, competition, predation and abiotic adaptation. Small tropical lakes of intermediate age are probably not biotically more complicated than similar-sized temperate lakes. The structure of the areas of adaptative radiation and the dispersal ability of the species are important for the present distribution of taxa. Fish play a key role in the tropics since many species both consume zooplankton and compete with them for algal and pelagic sestonic food. This important co-evolu-tion between fish and algae, leaving a fraction of the algal community with a predation refuge, may have decreased the ability of zooplankton to exploit algae. In addition, heavy predation from juvenile and adult fish may greatly simplify the zooplankton community, and have resulted in the scarcity of Cladocera, notably the efficient filter-feeder Daphnia. Little is known of possible physiological constraints to cladoceran distribution, however. Thus similar co-evolution as hypothesized between fish and algae seems not to have occurred to such a great extent between fish and zooplankton. Diurnal patterns in habitat selection of fish may also influence nutrient re-distribution in the tropics as in many temperate lakes. Serious environmental problems threaten tropical lakes, including eutrophication, clear-cutting of the rain forest, unwise introduc-tion of new species not adapted to prevailing conditions, overfishing, extensive use of biocids, and probably acidic rain in areas with poorly buffered waters. Important processes in tropical lakes could be elucidated by concentrating research upon the fate of phytoplankton successional production, involving competition, grazing, sinking, fungi and bacterial attack. Co-evolution of fish and algae should be further investigated as it could in part explain the general scarcity and simplicity of the zooplankton community. Limnocorral experiments should also be used for further assessing processes in tropical lakes.

Biological control of undesirable cyanobacteria in culturally eutrophic lakes

SummaryThe relationship between Oscillatoria agardhii and one of its natural grazers, the gymnostomid ciliate Nassula ornata was studied in laboratory experiments. Ingestion of Oscillatoria by Nassula increased exponentially over the temperature range 5–20°C, and the grazer was able to suppress Oscillatoria at all temperatures. Laboratory studies showed further that the predatory copepod Cyclops strenuus was able to suppress Nassula, and that juvenile roach (Rutilus rutilus) also fed on Nassula. In an enclosure experiment in an Oscillatoria-lake, Nassula reduced Oscillatoria concentration with more than 50% as compared to the control. The scarcity of Nassula in many Oscillatoria-lakes may be due to predation by cyclopoid copepods and juvenile fish; predominantly cyprinids. A proposed model describes well the population dynamics of Oscillatoria and Nassula in mixed cultures.

Restoring Daphnia lacustris G.O. Sars, 1862 (Crustacea, Anomopoda): a cryptic species in the Daphnia longispina group

While molecular markers have revealed several distinct species within the Daphnia longispina group, there is a need to reconcile these species with traditional nomenclature. Here we show that one such species, called D. longispina in recent literature based on molecular markers, can reliably be associated with the described taxon Daphnia lacustris G.O. Sars, 1862. Both mitochondrial and nuclear molecular markers readily distinguish this species from others in the D. longispina group. D. lacustris is absent in the region from which D. longispina was first described (Denmark), and the designation D. longispina must be reserved for another widespread species represented by Danish lineages. While the diagnosis of D. lacustris (and other species of the D. longispina group) by molecular markers is unequivocal, distinguishing it morphologically from other species is still problematic. The presently known distribution range of D. lacustris includes most of Norway, northern Finland and a single lake in the Polish Tatra Mountains. Its typical habitat is oligotrophic lakes without intense fish predation.

A geographical survey of littoral crustacea in Norway and their use in paleolimnology

Littoral Crustacea from 400 sites all over Norway were related to lake pH, conductivity, altitude and major biogeography. Being tolerant animals, littoral Crustacea do not fall into clear pH classes as do many Bacillariophyceae or Chrysophyceae. Littoral Crustacea were subdivided into 4 main pH classes: acidobiontic, acidophilous, circumneutral and indifferent, the last group containing most species. Species typical for acidic lakes in the region were: A. excisa, S. serricaudatus, A. curvirostris, A. nana, S. mucronata and D. brachyurum; whereas D. longispina, S. vetulus, A. intermedia and A. rectangula were more common in circumneutral waters. Many littoral Crustacea were shown to be related to altitude, which should be considered in studies on lake histories. Some species showed in addition a restricted geographical distribution. Conductivity, with values recorded in this study, played a minor role in distribution of taxa. More ecological studies are necessary to fully utilize the potential of cladoceran remains in understanding lake histories.

Diapausing fertilized adults

SummaryA lake population of the cyclopoid copepod Cyclops strenuus Fischer had a 1-year life cycle including a diapause from June to January. Dormant stages consisted of fertilized females, not previously described as a diapause stage, and a small fraction of cop V. The longevity of the sperm in a diapausing female was at least 6–7 months. Pre-fertilization is assumed to be an adaptive mechanism to assure fertilization when male abundance is low. This life cycle pattern also gives a shorter time-lag between diapause break and offspring development, and also the low total adult population exerts a lower predation pressure on their offspring. The flexibility of life cycle patterns of copepods is emphasized.

Major changes in pelagic rotifers during natural and forced recovery from acidification

Pelagic rotifers were studied in lakes with contrasting acidification histories situated in an acid-stressed region of southern Norway. Life histories and spatial distribution varied considerably between the investigated species, and influenced the recovery processes. Most headwater lakes have experienced strongly acidified environments during the last five decades, whereas lakes close to the Skagerrak coast have been stable within the same period. Rotifer diversity and abundance were reduced in the most acidic sites and increased towards the coast. Most surveyed species are known to possess sediment egg-banks, and after chemical recovery most rotifers dispersed into the plankton from these egg-banks and produced viable populations. Some species of the genera Polyarthra and Collotheca, and the species Kellicotta longispina and Keratella serrulata showed a striking ability to tolerate acidification, and were the dominant taxa in the acidmost environments. K. serrulata characterised, but did not numerically dominate, acid rotifer communities especially in the most coloured sites, and decreased following liming. The predominantly bacteriophageous genus Conochilus exploded in numbers shortly after liming, most probably because bacteria increased strongly during this transition phase. Planktivorous fish influenced indirectly rotifer abundance by consuming invertebrate predators and important rotifer competitors such as filter feeding cladocerans. Invertebrate predators, such as larvae of Chaoborus spp. and Heterocope saliens probably influenced rotifer distributional patterns in a complex top-down manner, both during chronic acidification and liming in environments with low fish predation. Important rotifer predators such as pelagic cyclopoid copepods, Bythotrephes longimanus and Leptodora kindti, were absent from the most acidic fishless lakes. Considerable populations of large-sized Daphnia longispina probably suppressed several rotifer species in sites with low fish predation, as did large populations of Bosmina longispina and Ceriodaphina quadrangula in lakes with intense fish predation.

Biomanipulation and food-web dynamics : the importance of seasonal stability

Responses of phytoplankton biomass were monitored in pelagic enclosures subjected to manipulations with nutrients (+N/P), planktivore roach (Rutilus rutilus) and large grazers (Daphnia) in 18 bags during spring, summer and autumn in mesotrophic Lake Gjersjøen. In general, the seasonal effects on phytoplankton biomass were more marked than the effects of biomanipulation. Primary top-down effects of fish on zooplankton were conspicuous in all bags, whereas control of phytoplankton growth by grazing was observed only in the nutrient-limited summer situation. The effect of nutrient additions was pronounced in summer, less in spring and autumn; additions of fish gave the most pronounced effect in spring. The phytoplankton/zooplankton biomass ratio remained high (10–100) in bags with fish, with the highest ratios in combination with fertilization. The ratio decreased in bags without fish to<2 in most bags, but a real grazing control was only observed in bags with addition ofDaphnia. No direct grazing effects could be observed on the absolute or relative biomass of cyanobacteria (mainlyOscillatoria agardhii). The share of cyanobacteria in total phytoplankton biomass was lowest in summer (7–26%), higher in spring (39–63%) and more than 90% in the autumn experiment. The development of the cyanobacterial biomass was rather synchronous in all bags in all the three experiments. A high biomass ofDaphnia gave no increase in the pool of dissolved nutrients in spring, a slight increase in summer and a pronounced increase in autumn. While a strong decrease in the P/C-cell quota of the phytoplankton was observed from spring to autumn, no effect of grazing or nutrient release could be related to this P/C-status. The experiments indicate that such systems, with high and stable densities of inedible cyanobacteria, are rather insensitive to short-term (3–4 weeks) biomanipulation efforts. This is supported by observations on the long-term development of the lake.

Acidification history and crustacean remains: some ecological obstacles

Crustacean remains are not yet useful as parameters for assessing the recent acidification phenomenon, due to too few studies on their ecology. The changes observed in the sediments are related not just to changes in lake chemistry, but also to predatory and competitive interactions. Moreover, the habitat pH of the organism may differ from the measured lake pH, and may as such not be representative. Therefore, species in some acidic lakes may decrease their abundance, whereas the same species in nearby acidic lakes increase. The caution mainly concerns species with no clear pH distribution. Species predominant in acidic or alkaline lakes generally follow patterns more closely related to their physiology. Study of pelagic species, including both body size and shape measurements, should be included in the analysis. If all the ecological factors are included, animal as well as algal remains would form powerful tools to reveal lake histories.

Acidification in southern Norway: seasonal variation of aluminium in lake waters

Total aluminium, pH and water colour were measured over two years in three lakes of different pH in an area undergoing anthropogenic acidification in southern Norway. Al content decreased in Ca-rich lakes near the coast, but also with increasing organic content of the lakes. Seasonal variation was pronounced in the most acidic lake. Al concentration seems virtually independent of pH, suggesting that it is in the form of organic complexes, at least in the two most coloured lakes. Precipitation processes affecting fish metabolism in the acidic lakes with high Al content may have caused the frequent lack of success in liming lakes in this area. Similarily, humic acidic lakes with organic complexed Al still carry considerable fish biomass.

A simple method for maintaining a laboratory supply of limnetic cyclopoid copepods

The following method is used for maintaining a laboratory supply of freshwater cyclopoid copepods with intervening diapause phase. Animals are collected with a core sampler from the upper sediment surface just after initiation of diapause, i.e. during early summer for summer resting species and mid autumn for winter resting species. The method makes it possible at all times of the year to keep a variety of cyclopoid copepod species ready for experimental work. The following widespread species have been used: Mesocyclops leuckarti, Thermocyclops oithonoides, Diacyclops bicuspidatus, Cyclops vicinus, C. strenuus and C. abyssorum. Probably all cyclopoid copepods with a diapause phase could be stored according to the present method.