Factors controlling the summer development of copepod populations in the southern bight of the North Sea

In two consecutive years an intensive sampling programmed was implemented at a fixed station in Dutch coastal waters to obtain a detailed record of the summer development of copepod populations in relation to phytoplankton and macroplankton abundance. The central question was whether densities of copepods are controlled by predation, in particular by invertebrate pelagic carnivores, or by food limitation. Methods applied to estimate daily predation by observed stocks of carnivores included analysis of gut contents and digestion rate, extrapolation of experimental feeding rates and of literature data on daily rations and maintenance needs. Chlorophyll-a and cell concentrations served as a rough measure for algal food supply.Since a decline in copepod densities manifested itself most clearly in decreasing naupliar numbers in both years, populations were assumed to be regulated mainly by recruitment or survival of these early life stages. Naupliar declines coincided with maximum densities of the hydromedusa Phialidium hemisphaericum, which dominated the macroplankton both in abundance and in biomass and reached a maximum density of 467 specimens·m⁻³ or 7 mg C·m⁻³. Copepod eggs appeared by far the most frequent prey item in their guts. However, these eggs are digested very slowly, if at all, and may often be ejected without any visible damage. The effect of egg predation on naupliar recruitment seems therefore relatively unimportant. Predation on swimming copepod stages was generally low. There was no evidence of selective feeding on nauplii. The maximum values of calculated predation pressure exerted by Phialidium populations matched daily copepod production only by way of exception. Impact of other invertebrate carnivores was negligible. As predation did not play a significant role, food availability seems the key factor underlying copepod population dynamics. The consequences of food limitation (reduced egg production, production of diapause eggs and enhanced cannibalism) are discussed. The observed coincidence of maximum predator abundance and minimum chlorophyll-a and diatom concentrations does not support the hypothesis that carnivores are able to indirectly benefit phytoplankton growth by reducing grazing pressure of herbivores.     

Denitrification and nitrous oxide in the North Sea

In situ sediment denitrification rates were determined in the major areas of deposition of the North Sea, using the acetylene block technique. In addition, nitrous oxide profiles of the water column were determined. Nitrous oxide production generally occurred in the photic zone possibly due to nitrification; and throughout the water column in the German Bight region. Consumption at depth was possibly due to reduction in the anoxic microzones of faecal pellets, concentrated at the thermocline. Saturation of surface waters was 102.2% compared to 130.3% in the German Bight region. Calculated flux of nitrous oxide to the atmosphere was 9.5 × 10⁶ kg yr⁻¹, over half of which was produced in the German Bight.Sediment denitrification rates varied through three orders of magnitude; the highest value of 150 μmol m⁻² d⁻¹ was recorded in the Norwegian Trench. Nitrous oxide production by the sediments was low (1.1 μmol m⁻² d⁻¹ max.), and was undetectable at half of the sites. Sediment nutrient profiles exhibited porewater nitrate concentrations exceeding that of the overlying water suggesting that denitrification was fuelled by nitrification, which, in turn was related to other environmental variables. A significant positive relationship existed between in situ denitrification rate and the nitrate content of the upper sediment. Extrapolation of the rate to the total area of deposition in the North Sea suggests that denitrification is responsible for a minimum loss of 7.5–12% of the total annual nitrogen contaminant input.     

Organochlorines in benthic polychaetes (Nephtys Spp.) and sediments from the southern North Sea. identification of individual PCB components

ΣPCB concentrations (as the sum of concentrations of 35 individual components) on a lipid basis varied more than an order of magnitude between samples of polychaete worms (Nephtys spp.) from different stations in the southern North Sea. PCB patterns, i.e. the relative contribution of each component to ΣPCB, were highly similar for the larger part of the area.The geographical differences between concentrations of PCB's reflected the general routes of residual water and suspended particulate matter transport throughout the North Sea. The Rhine is likely to be the main source for PCB's in Dutch coastal waters, including the entire coastline formed by the Dutch Wadden Sea islands. Theoretical patterns of PCB's in solution were calculated from the PCB patterns in Nepthys and values of bioconcentration factors reported.Concentrations of HCB, α- and γ-HCH, dieldrin, p,p′-DDD and p,p′-DDT were at least an order of magnitude lower and close to, or even below, detection limit in most samples. Although concentrations of α-and γ-HCH in solution were the highest of the compounds investigated, their concentrations in Nephtys were among the lowest, which might have been due to their relatively high solubility in water, or metabolism to more polar derivatives.Based on total sediment analyses, organochlorine concentrations in Nephtys spp. were not correlated with the concentrations in the (type of) sediment which they inhabited. In sediments the fraction of grains <63 μm appeared to be very important.A sewer outlet near Grimsby (U.K.) was found to be a source for HCB and Σ-DDT.     

Inshore and offshore diversity of epibenthos dredged in the North Sea

Large epibenthos (>1 cm.) was dredged in the German Wadden Sea and in adjacent offshore areas at 10 to 30 m depth in Helgoland Bight. There were apparent differences in species composition and relative abundances between the 2 regions. The hypothesis of increasing species richness in offshore direction and of increased individual numbers inshore was tested. Neither proposition was found to be generally true.Regional species number was higher offshore (85) than inshore (73). However, a direct comparison of 11 + 11 localities and of 66 + 66 dredge hauls revealed no significant differences. Ratios of species richness for localities versus regions and for hauls versus localities differed significantly, indicating a higher regional heterogeneity offshore and a higher habitat heterogeneity inshore. Thus, causes for an overall similar diversity were found on different spatial dimensions.Few species were encountered on bottoms subject to the riverine waters of Elbe and Weser. Many decapod crustaceans attained stronger populations inshore, while ophiuroids completely dominated the epibenthos offshore and caused higher individual numbers there. Historical sources suggest that the epibenthos was richer in the past than it is now.     

Growth and dynamics in populations of echinocardium cordatum living in the North Sea off the Dutch North coast

Numerical densities and size distributions in populations of the spatangoid Echinocardium cordatum were followed at 19 fixed sampling stations with sandy sediments at depths of 6 to 18 m off the west-Frisian islands of Texel and Terschelling.Recruitment was irregular with some recruitment occurring during half of the years and successful year classes developing during only 3 of the years in the 10 year period of observation (1972–1982). Annual mortality of the strong year class born in 1972 was rather regular in the ages of 1 to 7 years, averaging ∼27% in the 2 areas, but was followed by catastrophic mortality at most stations during the severe winter 1979.Growth was rapid. A mean length of 2 to 2.5 cm was reached after 2 growing seasons and 4 cm after 4 or 5 growing seasons.     

Seasonal differences in concentrations of particulate lipids,proteins and chitin in the North Sea

The concentrations of particulate lipids, proteins and chitin were determined in the North Sea in January 1992, October 1992 and March 1993, with additional measurements of total suspended matter concentrations and organic matter contents in the suspended matter (POC) by ashing and wet-oxidation. During each of the three cruises, the proteins were by far the most important of the three components measured, followed by the lipids. The concentrations of chitin were always very low. The concentrations of these three components were highest in March and lowest in January; there was also a variation in the distribution over the North Sea between the three cruises. In January 1992, the suspended matter contained more organic matter in the northern North Sea than in the southern North Sea, but the higher concentrations of total suspended matter in the southern North Sea resulted in an inverse distribution for the concentrations of organic matter per volume of water. In March 1993, highest concentrations of the three organic compounds were observed in the Skagerrak and along the eastern side of the North Sea, decreasing towards the west. The suspended matter was richest in organic compounds in the Skagerrak and in the Norwegian Channel. In October 1992, the situation was very similar to that observed in January 1992, but the concentrations of organic compounds in the suspended matter and per volume of water were higher. The ratio of proteins to lipids was similar to this ratio in phytoplankton. The composition of the particulate organic matter became very close to that of phytoplankton if we assume that we missed a large amount of carbohydrates; the small differences persisting were probably due to the contributions of zooplankton, bacteria and terrestrial sources. The total contents of particulate lipids + proteins + chitin were low, but when estimated amounts of carbohydrates were added, the total particulate organic-matter concentrations found were very similar to the total particulate organic carbon concentrations found by wet-oxidation. During the period of sampling, no large concentrations of phytoplankton occured.     

Nutrient dynamics in the North Sea: Fluxes and budgets in the water column derived from ERSEM

Nutrient dynamics for phosphate, nitrate, ammonium and silicate have been simulated with ERSEM, the European Regional Seas Ecosystem Model. From the model results budgets for the dissolved inorganic nutrients and the corresponding particulate fractions have been calculated. The annual cycles of the nutrients phosphate and silicate compare quite well with the observed ranges of variability. This does not hold for ammonium and nitrate. Biologically mediated transformations such as nutrient uptake and pelagic and benthic mineralization are the dominant processes in changing the nutrient concentrations with the horizontal advective contributions playing a minor role during the productive season.Vertical advection and vertical diffusion have a clear seasonal signal, with a maximum in February. The decay of the advective nutrient transport in summer is caused by the depletion of the upper layer of dissolved inorganic nutrients by algal uptake. The inflow of nutrients in the northwest is almost balanced by the outflow in the northeast, without causing large nutrient transports into the shallower areas from the north. However, from the coastal areas there is a nutrient flow towards the central North Sea, enhancing primary production in the central area.     

Late Pleistocene and Holocene deposits in the witch and fladen ground area,Northern North Sea

Determinations of carbonate, total N, and organic C content, grain size distributions, analyses of Foraminifera, molluscs and pollen, as well as two ¹⁴C data of cores taken in the Witch and Fladen Ground area, were used to supplement earlier interpretations of acoustical reflection records, in an attempt to clarify the Late Pleistocene and Holocene history of the northern North Sea.Starting from below, the Lower and Upper Swatchway Beds, containing not only glaciomarine deposits but also temperate marine sediments, are shown to be of at least Middle Weichselian and possibly of Eemian age. The overlying Hills Deposits consist of a subglacial till, forming the base of a morainic facies, with arctic glaciomarine sediments above and alongside it. The postglacial rise of the sea level and the increase of sea water temperature were recorded in the glaciomarine Fladen Deposits and marine Lower Witch Deposits. For the greater part sedimentation ended during Boreal times (pollen zone IIa2) with the formation of the Upper Witch Deposits, a veneer of fine sands which is correlated with the development of the East Bank Deposit northeast of Dogger Bank. The succession is correlated with a sequence of deposits in a system of buried channels between Witch Ground and Dogger Bank.A belt of tunnel valleys marks the maximum Weichselian ice extension, which occurred at roughly the time of formation of the Hills Deposits. A Witch Ground shore line, probably of about 15 000 y BP, is present at approximately 110 m below sea level, and is proposed to represent the lowest level of the last Weichselian substage.     

The inhibition of growth by sub-optimal temperatures and the potential for selection at the cellular level

Summary At present research into the inhibition of plant growth by sub-optimal temperatures is concerned with the examination of a large number of factors. No single aspect of the response to temperature has been found to be of paramount importance. As a consequence selection for low temperature tolerance is complicated.Here a new approach is proposed in which inhibition of cell division is regarded as a limiting process in growth under sub-optimal temperatures, and the role of other processes as the supply of substrate for cell division. This approach may be of use in the examination of the response of plant growth to temperature and could form the basis of a method for screening and selection for cold tolerance using cells in culture.Guestworker Project Group Tomato Physiology.     

Phytoplankton biomass and potential nutrient limitation of phytoplankton development in the southeastern North Sea in spring 1985 and 1986

The vernal phytoplankton bloom was observed during cruises to the southeastern part of the North Sea (east of 6°20' E, south of 56°50'N) in 1985 and 1986. Maximum phytoplankton biomass expressed as phytoplankton carbon was similar in both years (14.5 and 17 g PPCm⁻² respectively). In 1985 the bloom was located in the less saline coastal water close to the North Frisian coast. Phytoplankton was dominated by Coscinodiscus concinnus and Thalassiosira nordenskiöldii. In 1986, highest phytoplankton biomass was observed northwest of the island of Sylt, where Thalassiosira nordenskiöldii was the dominant phytoplankton species. Within the areas of high phytoplankton standing stock, concentrations of the inorganic dissolved nutrients phosphate and silicate had dropped to nearly undetectable concentrations, whereas both in 1985 and 1986 the water was still rich (10 – 20 μM.dm⁻³) in inorganic nitrogen (DIN). This, as well as the high ratios of DIN:PO₄ and DIN:Si(OH)₄ (> 50) point to phosphate and silicate as potential limiting nutrients during the spring phytoplankton bloom. The ratios of total nitrogen (TN) to total phosphorous (TP) (> 30) indicate also that phosphorous might then be in short supply. Phosphate and silicate have to be considered as potentially limiting nutrients during the vernal plankton bloom in the coastal waters of the southeastern North Sea, with nitrogen being present in surplus at that time of the year. However, in the more offshore areas nitrogen may be considered the potentially limiting element at the same time.     

Seasonal fluctuations in vertical distribution and breeding activity of a subtidal harpacticoid community in the southern bight,North sea

Seasonal changes in vertical distribution and breeding activity of 14 dominant harpacticoid species from a sandy station in the Southern Bight of the North Sea are described over a period of 1 year, from January 1983 to January 1984.It is clear that the bulk of the meiofauna penetrates deeper than expected from the literature. The need for long cores for quantitative work in subtidal sandy areas is stressed.The factors affecting vertical distribution in the sediment are discussed. Except for the winter breeder Interleptomesochra eulittoralis, all other species bred at least partially in summer when temperature is high. In these species migratory behaviour and reproductive activity were interrelated, and as a result egg development took place in the surface layers of the sand.Only Microarthridion littorale bred continuously throughout the year. In all other species the breeding season, as defined by the simultaneous presence of juveniles and ovigerous females in the population, was either restricted or protracted: distinct maxima occurred at different times in different species, but the majority reached maximum breeding activity in the summer.The observed breeding patterns of the dominant species are compared with those reported from other localities. Most of them are so-called “plastic species”, which show a latitudinally related alteration in breeding periodicity.     

Changes in growth of plaice Pleuronectes platessa L. and sole Solea solea (L.) in the North Sea

The changes in growth of plaice and sole between 1957 and 1988, as estimated from samples of the commercial fishery and pre-recruit surveys, were analysed in order to study possible density dependent effects. Indices of potentially competitive biomasses of plaice and sole, based on Lloyd's index of mean crowding, were estimated from the average spatial distribution of various age groups during the summer growing period and from the population age structure as estimated by virtual population analysis.Growth of all age groups of sole increased in the 1960s and was stable in the 1970s and 1980s. In plaice only age groups 1 to 3 showed a similar increase in the 1960s, whereas the growth of 1-year-old plaice tended to decrease in the 1980s. Growth did not show a negative correlation with mean crowding, except in age group 1 of plaice and in age group 3+ of sole. It is concluded that these negative correlations do not provide unequivocal evidence for density dependent growth in plaice and sole, since they could equally well be caused by parallel but unrelated trends in time of one or more other factors. The simultaneous increase in growth in the 1960s of age groups of sole and plaice in the southern North Sea, and the absence of such an increase in age groups in the central North Sea, suggests that food availability must have increased in the Southern North Sea. This inference is supported by several macrobenthos studies. Whether the reduced growth of 1-group plaice in the 1980s, when recruitment was well above the average level, is caused by density dependent growth or to a reduced food availability remains an open question.     

On selective tidal transport in the migration of North Sea Plaice (pleuronectes platessa) and other flatfish species

Evidence is given for selective tidal transport in North Sea plaice and other flatfish species. Data collected during midwater plankton surveys in two Dutch Wadden Sea inlets in early spring in the years 1970 to 1977 show that plaice, sole, flounder and dab demonstrate pelagic swimming behaviour in that period. Plaice, sole and flounder show a clear selective tidal transport mechanism to re-enter the Wadden Sea after staying in the open sea in winter whereas dab tends to leave the Wadden Sea. Data on by-catches of semi-pelagic pair trawl fishery for gadoids in the Dutch coastal area show that small quantities of plaice may be caught off the bottom throughout the whole year whereas sole moves to higher levels in two distinct periods only. The first period lower left diagram) demonstrate a strong preference for northgoing currents. This is also the direction towards the areas from which the plaice used had been transported to the laboratory and therefore this preference for northgoing tides might be interpreted as a clear case of homing to the Silverpits and Borkum areas.Also the rate of dispersion of the cluster of displaced tagged plaice in general (Fig. 18, lower right diagram) is much faster in the first months after release than that of the local plaice. Thus displaced plaice are more strongly motivated to migrate than non-displaced fish. This may well have been the case too in the plaice equiped with the transponding acoustic tags. This together with the release in an area with strong currents and narrow tidal ellipses means that the plaice should be considered to have been in an exceptional situation, extremely suited for travelling at great speeds in a fixed direction. Fig. 18 also shows the average surface tidal current velocities, derived from the 13 maps with hourly data on water circulation in the Stroomatlas Nederland (1963). It is obvious that the Lowestoft plaice, mostly released nearer to the coast within the release area, encountered the highest average current velocities observed in the North Sea. It cannot be proven that these circumstances explain completely the vast speed of the Lowestoft plaice but it certainly demonstrates that there is a bias in these experiments which could easily affect the results. When drawing conclusions in respect to the “normal” picture of plaice migration this bias should not be ignored.In the common sole selective tidal transport is observed in juveniles when entering the Wadden Sea in spring. In adult soles this mechanism is a common feature in March to May during the spawning migration but no evidence for the use of this mechanism is available during other periods of the year. The sole thus is an example of a flatfish species using selective tidal transport in a restricted period and for special purposes only.In plaice the use of selective tidal transport in the Wadden Sea inlets has been shown for juveniles. In adult plaice the need for using the mechanism might be much less although plaice have been seen swimming at the surface in spring (van den Ende, 1849) during day time quite in contrast with their normal behaviour. In spring nights, among soles swimming at the surface, plaice too have been observed (together with some dabs and turbots) but in much lower numbers than the sole (de Veen, 1967).On the whole there is not much evidence for a strong continuous use of selective tidal migration in plaice. No serious drops in the catch rates during specific tidal phases in the bottom trawl fisheries have been recorded by the results of the UK 81 otter trawl fishery. The coincides with the re-entering of soles in the Wadden Sea and with the well-known phenomenon of soles swimming at the surface. During this period the soles use selective tidal transport to reach the spawning grounds near the continental coasts.Data on catch per unit effort of a Dutch ottertrawler in 1958 to 1960, constantly fishing in one restricted area near Terschellingerbank lightvessel, reveal significant differences between day and night catch rates and a change from a preference for flood tide in the first half of the year to a preference for ebb tide in the second half.The available evidence for selective tidal transport in plaice is discussed in relation to the patterns of migration in 4 plaice sub-stocks studied by means of tagging experiments, and in relation to results of displacement experiments with mature and immature plaice. The tagging and displacement experiments present a picture of migration in which the directional swimming is a weak component superimposed on largely random movements and the speeds of travelling are low. The plaice react to displacement by trying to return to their original grounds and this tendency to home lasts for a full year.This picture about selective tidal transport contrasts with results obtained by the Lowestoft Laboratory. Plaice equiped with transponding acoustic tags did show largely directed and fast tidal migration before and after spawning. Arguments are given that these plaice were, however, in an exceptional position and may have shown homing behaviour. Notwithstanding this reservation to the Lowestoft experiments, these results and the present ones are explained if it is assumed that apparently only a fraction of the population uses the mechanism of selective tidal migration, thus saving energy and achieving quick transport when necessary. It is likely that in plaice as in sole the mechanism will be used in special cases only, probably for short periods, in certain areas.     

Chemical and physiological changes of phytoplankton during the spring bloom,dominated by phaeocystis pouchetii (Haptophyceae): observations in Dutch coastal waters of the North Sea

During the spring bloom of phytoplankton in Dutch coastal waters of the North Sea the flux of ¹⁴C-labelled products into pools of protein, polysaccharides, small metabolic intermediates and lipids was studied. The molecular size distribution of extracellular products, amounting to 28% of the particulate production, was determined by gel filtration. Concentrations of protein and carbohydrates were determined in the same phytoplankton samples. At an early stage of the bloom protein synthesis and protein concentration dominated over carbohydrate synthesis and carbohydrate concentration, but after the peak of the bloom carbohydrates were the predominant photosynthetic endproduct. These observations have been compared to similar observations on unialgal cultures of Phaeocystis pouchetii. The influence of phosphate depletion on the extracellular release of organic compounds and the impact of the colonial growth form of Phaeocystis pouchetii on the distribution of intra- and extracellular labelled compounds are also discussed.     

Genotype × environment interaction amongst spring barley lines grown at sites in the northern and southern hemispheres

Summary Measurements and scores for juvenile plant habit, time to ear emergence, plant height, leaf diseases and 1000 kernel weight were recorded on 123 lines of spring barley grown at Palmerston North, (40°S) New Zealand, and at Cambridge, (52°N) England. The material was grown at both sites from F₄ to F₇.For juvenile habit, time to ear emergence, plant height and 1000 kernel weight highly significant correlations were found between sites and seasons though few accounted for more than 50 per cent of the variation. There was no evidence of closer correlation between seasons within sites than between sites and seasons, and it was concluded that selection at either site should be equally effective.Analysis of yield performance over sites and seasons within sites revealed large effects due to genotypes and seasons within sites but small effect due to sites. Large interactions were found between genotypes and sites but the genotypes × seasons within sites effect was relatively small; better agreement was found in relative yields between seasons within sites than between sites, so that yield performance in one country was a poor indication of potential performance in the other country.The implications of these results for the use of a selection nursery in New Zealand are discussed.     

Free-living plathelminthes in sheep-grazed and ungrazed supralittoral salt marshes of the North Sea: Abundance,biomass, and their significance in food chains

The supralittoral salt marshes of the North Sea are marked by high halophyte primary productivity. The environmental factors are strongly fluctuating. Despite these features the metazoan meiofaunal abundance is equal to that found in other littoral habitats. On average 1250 marine metazoans are found per 10 cm² in ungrazed and 770 per 10 cm² in sheep-grazed supralittoral salt marshes. Nematoda dominate in numerical abundance, Oligochaeta in biomass. Plathelminthes account for 15% of marine metazoans in ungrazed and 5% in grazed salt marshes.Total plathelminth abundance increases with halophyte density, whereas the abundance of diatom-feeding Plathelminthes decreases. In ungrazed marshes on average 104 Plathelminthes are found per 10 cm², accounting for a biomass of 0.65 g DW·m⁻². In sheep-grazed marshes the average abundance is only 32 individuals per 10 cm², accounting for a biomass of 0.1 g DW·m⁻². Average individual weight is 3.2 μg DW or 2.5 μg AFDW.In grazed salt marshes, 30% of plathelminthes feed on diatoms, 66% are predators, and 4% feed on bacteria (gut analysis). In ungrazed salt marshes only 3% are diatom-feeders, and 90% are predators feeding on Nematoda, Copepoda, Oligochaeta, and smaller Plathelminthes. Presumably plathelminthes are top predators on the salt marsh meiofauna.     

Cyclic organochlorines in the hermit crabs Pagurus bernhardus and P. pubescens from the North Sea. A comparison between winter and early summer situation

The body burdens of 5 cyclic organochlorine compounds were determined in abdomens of Pagurus bernhardus and P. pubescens from the North Sea during two seasons. The seasonal bioaccumulation of ΣPCB (as the sum of concentrations of 24 individual components) is influenced by the substances associated with food originated from the spring plankton bloom and the resuspended sediments during the winter.The patterns of individual PCB congeners depended on the weight of the abdomens, and the geographical position of sampling. In offshore samples, the findings indicated a relationship between the weight of the abdomen (roughly the age) and the bioconcentration of higher chlorinated biphenyls. The results from the coastal areas demonstrated that the uptake by food determines the steady state concentration of the higher chlorinated congeners in the hermit crab. Pagurus pubescens probably possesses a mixed-function oxygenase (MFO)-system different from that of Pagurus bernhardus because the congeners 2,2′, 3,3′, 4,5,6′-heptachlorobiphenyl (174) and 2,2′, 3,3′, 4′, 5,6-heptachlorobiphenyl (177) were absent in all samples of the former species but were always present in the latter.The geographical differences between concentrations of ΣPCB reflect the general mean-routes of water and suspended particular matter transport through the North Sea: the Southern Bight and the continental coasts were more polluted with PCBs than the central North Sea. Maximum concentrations of p-p′-DDE were found in the southern parts and parts of the central North Sea. Samples from the German Bight showed the highest concentrations of Lindane. The results indicate that the atmospheric transport of lower chlorinated biphenyls, HCB, α-HCH and Lindane may be important in the distribution of these components, because they only showed slight gradients or even none at all in the area investigated.     

Oxygen regime,respiratory activity and biomass of microorganisms,and the carbon budget in the Fladen Ground area (Northern North Sea) during spring

A watermass, on the Fladen Grounds, marked with a drifting buoy, was studied daily during one month in spring. Hydrographic parameters, nutrient regime, oxygen regime, biomass of microorganisms and respiratory activity were determined; the high variability of the observations is discussed. From these observations, and earlier publications on the same cruise, a carbon budget is calculated. From oxygen flux calculations it appeared that the net ecosystem production must be more than 2.5 g C m⁻².d⁻¹ which could not be verified by C-14 primary production measurements. Presumably high variability in the system in time and space makes it impossible to get a fitting budget.     

Breeding for oil yield and short oil palms in the second cycle of selection at La Dibamba (Cameroon)

Breeding in oil palm aims mainly at improving palm oil yield. Short palms are also desired because they are easy to harvest and increase the economic life of the plantation. A total of 23 progenies from Deli × La Mé and Deli × PO 1097 P (Yangambi) crosses were field tested at La Dibamba Oil Palm Research Centre in Cameroon from 1988 to 1998. Oil yield components were measured on per palm basis from 3 to 9 years after planting, and the vertical growth at 6 years after planting. The mean oil yield of the trial was average, representing 102% of the control (3.515 t/ha), and the mean height 6 years after planting was 101% that of the control (88.0 cm). The analysis of variance detected differences among progenies for various yield parameters and vertical growth. The comparison of means showed clear separation of groups of progenies for oil yield at the juvenile period (3–5 years after planting). At maturity (6–9 years after planting), the groups of progenies were overlapping indicating that progenies were comparable each other. However, four precocious progenies and high oil yielding at maturity (119–122% of the control) in absolute values can be released to planters. Though their vertical growth represented 116–127% of that of the control, short pisifera palm are expected from parental selfs or crosses given the intrinsic heterozygosity of oil palm.