Planktonic Development Research Articles

EFFECTS OF SALINITY AND STARVATION ON LARVAL DEVELOPMENT OF THE CRABS ARMASES RICORDI AND A. ROBERTI (DECAPODA: GRAPSIDAE) FROM JAMAICA, WITH NOTES ON THE BIOLOGY AND ECOLOGY OF ADULTS

ABSTRACT Adult ecology, egg production, larval development, salinity tolerance, and starvation resistance of the grapsid crabs Armases ricordi and A. roberti were studied in Jamaica. The species have different adult habitats. Armases ricordi lives in dry hot habitats up to 200 m from the shore, whereas A. roberti lives on the banks of fresh-water rivers up to 9.6 km from the sea, a relatively moist habitat with moderate temperature variations. Newly deposited eggs were significantly larger in A. roberti (0.40 mm) than in A. ricordi (0.35 mm). Both species produced many eggs per clutch, more than one clutch in an intermolt stage, and at least one clutch of eggs was fertilized without remating. Ovigerous females of A. ricordi migrate to the sea for larval release, whereas A. roberti apparently release the larvae into the river to be transported to the sea. In both species, larval development takes place in the sea and consists of 4 zoeal stages. Duration of larval development did not differ between A. ricordi (16.1 days) and A. roberti (16.9 days), but the megalopa stage in A. roberti (12.0 days) was longer than in A. ricordi (9.1 days). In both species, larvae depended on food intake in order to complete development, and the duration of survival in starved larvae varied significantly between hatches. Larvae of Armases roberti, hatched with a larger body size, showed a higher endotrophic potential and the first crab stage was larger than in A. ricordi. The range of salinity tolerated for development to the megalopa was 20-45 ppt in A. ricordi and 15-45 ppt in A. roberti. The species differed greatly in salinity tolerance in the first zoeal stage, with 100% of the larvae surviving fresh water only for 1 h in A. ricordi, but for 2.5 days in A. roberti, allowing the first zoea of the latter to be transported to the sea from distant inland habitats. The differences between A. roberti and A. ricordi in endotrophic potential, body size, and fresh-water tolerance of the first zoea are likely to be related to the larger egg size and, hence, larger yolk provisioning per egg in A. roberti. We discuss the significance of migration and larval release pattern in relation to inland distribution of adults for species with marine planktonic development.

Variability in life history characters of the Chilean oyster Tiostrea chilensis (Philippi, 1845)

Differences in some life history characters of the Chilean oyster, Tiostrea chilensis, were examined for three populations in the northern North Island of New Zealand. Previous studies of oyster populations in the South Island have predicted that populations at lower latitudes would produce smaller‐sized larvae and substantial numbers of planktonic larvae. For only one northern population was there some evidence of the release of planktonic larvae. However, larvae from all three northern study populations were smaller in size than reported for all other locations, both in New Zealand and Chile. This was because of a trend for larval size to vary inversely with sea water temperature. The results of this study indicate that life history characters of this species can vary greatly at both the individual and population level. Furthermore, this variability does not always conform to commonly proffered life history principles which assert that planktonic development and small larvae become more common at low latitudes.

MOLECULAR PHYLOGENETIC ANALYSIS OF LIFE-HISTORY EVOLUTION IN ASTERINID STARFISH.

We analyzed phylogenetic relationships among 12 nominal species of starfish in the genera Patiriella and Asterina (Order Valvatida, Family Asterinidae), based on complete sequences for a mitochondrial protein coding gene (cytochrome oxidase subunit I) and five mitochondrial transfer RNA genes (alanine, leucine, asparagine, glutamine, and proline) (1923 bp total). The resulting phylogeny was used to test a series of hypotheses about the evolution of life-history traits. (1) A complex, feeding, planktonic larva is probably ancestral for these starfish, but this is not the most parsimonious reconstruction of ancestral larval states. (2) The feeding larval form was lost at least four times among these species, and three of these losses occurred among members of a single clade. (3) Small adult size evolved before both cases of hermaphroditism and viviparous brooding, but viviparity was not always preceded by an intermediate form of external brooding. (4) An ordered transformation series from feeding planktonic development to viviparous brooding has been predicted for starfish, but we could not find an example of this transformation series. (5) Viviparity evolved recently (< 2 Mya). (6) Both species selection and transformation of lineages may have contributed to the accumulation of species with nonfeeding development among these starfish. (7) Neither Asterina nor Patiriella are monophyletic genera. Larval forms and life-history traits of these starfish have evolved freely under no obvious constraints, contrary to the widely assumed evolutionary conservatism of early development.

Microgeographical shell variation in Littorina striata , a planktonic developing periwinkle

Littorina striata is a strictly Macaronesian, intertidal periwinkle with planktonic development. The species produces both nodulose and smooth shells, which co-occur at Ilheu de Vila Franca do Campo, a drowned crater situated about 1000 m off the south coast of Sao Miguel, Azores. The present work describes and analyzes the shell variation, temporal change and ecological distribution of the two shell types at this crater over a 3-year period. Nodulose shells were more common in the sheltered lagoon inside the crater, while smooth specimens dominated the outside of the wave-exposed crater. Moreover, nodulose specimens were smaller and weighed less compared to smooth ones. However, regardless of morphotype, shells from the lagoon had a smaller aperture and were less globose than those from the outside. Within an exposure regime, smooth specimens had a larger aperture. These patterns remained constant over time. They are tentatively interpreted as functional adaptations to thermal stress and wave exposure. The mechanisms that maintain these patterns are still unknown. Yet, aperture height of specimens transplanted from the lagoon to the outside increased markedly over a period of 5 months, whereas no comparable changes were observed in other experimental groups (i.e. transplanted from the outside into the lagoon, transplanted within the outside and transplanted within the lagoon; the latter two transplant groups being the blancos). This suggests that at least the aperture size in L. striata may be an ecophenotypically plastic trait.

Comparison of phytoplankton succession in two lakes of different mixing regimes fig: 12 tab: 4

The influence of frequent water-column mixing on phytoplankton succession and species composition was studied by comparing plankton dynamics in two lakes (Lake Opinicon and Upper Rock Lake) which have similar water chemistry, but differ widely in their basin morphometries: Opinicon is shallow and frequently-mixed during the ice-free season, whereas nearby Upper Rock is deep and dimictic. The species composition and the seasonal succession of phytoplankton were broadly similar in both lakes over the three years of study. The spring phytoplankton in both lakes consisted of diatoms that were replaced in the summer and fall by cyanobacteria. Diatom peaks in Opinicon preceded similar peaks in Upper Rock. Loss of diatoms from the phytoplankton in Upper Rock appeared to be related to sedimentation; in contrast, zooplankton herbivory may have resulted, partly, in loss of diatoms in Opinicon. Lightly-silicified and spindle-shaped diatoms (Rhizosolenia and single-celled Fragilaria) were more abundant in the dimictic lake, whereas heavily-silicified diatoms (Aulacoseira) were more abundant in the frequently-mixed lake. Zooplankton taxa and their seasonal patterns were similar in both lakes, but populations were considerably denser in the frequently-mixed lake. The results of canonical correspondence analysis and detrended correspondence analysis (multivariate ordination techniques) indicated that, although phytoplankton growth was influenced by similar physical and chemical variables in both lakes, the influence of chemical variables was stronger in Upper Rock. A frequently-mixed system, such as Lake Opinicon, appeared to provide a similar environment for plankton development to a well-mixed epilimnion of a typical dimictic lake, such as Upper Rock Lake.

Dissolved organic matter and its utilization by bacteria during spring in the Southern Ocean

Concentrations of dissolved organic carbon (DOC) and nitrogen (DON) were measured during early austral Spring 1992 at a number of stations along the 6°W meridian between 47° and 60°S. This included the Polar Front in the north, the zone of melting sea-ice in the south, and waters of the Antarctic Circumpolar Current in between. Concentrations of DOC were low in deep water (34–38 μM) with generally similar or slightly higher values in the surface mixed layer (38–55 μM). DOC:DON ratios are wider in surface water than in deep water, i.e. surface accumulations contain relatively C-rich dissolved organic matter. The highly variable distribution of the surface DOC was not related to hydrographic or biotic features (fronts, plankton development) indicating the lability and transient occurrence of this material. Growth rates of bacteria were determined in subsamples from 51 0.8-μm-filtered batches of seawater incubated in the dark at in-situ temperature. Thymidine and leucine uptake and bacterial biomass change as well as changes in dissolved organic carbon in the batches, and oxygen consumption in parallel incubations correlated linearly over 2 weeks of incubation which allowed extrapolation to in-situ conditions. Bacterial growth in these experiments depended strongly on the amount of initial DOC. Growth in water from greater depth (1000 m) containing 38 μM DOC was minimal, as were DOC-decrease and oxygen consumption. Higher rates were observed in surface water slightly enriched with DOC, and highest rates in surface water amended with DOC-rich melted sea ice. Bacterial growth efficiencies (biomass C-increase vs DOC consumed) were about 30%. The experiments showed that at least 40–60% of the DOC in excess of deep water concentrations was available to bacteria.

Fish Larvae Abundance and Distribution in the Coastal Zone off Terminos Lagoon, Campeche (Southern Gulf of Mexico)

Fish larvae abundance and distribution in the coastal zone off Terminos Lagoon and their relation to the environmental features of the Lagoon inlets were analysed (1986–87). The sampling grid consisted of 24 stations extending between 0·5 and 10 km off the Lagoon, including both Terminos Lagoon Inlets; El Carmen and Puerto Real. A total of 23 families and 43 species were identified. Highest larval abundance was registered during the rainy period (July and September) when the fluvial discharges favoured the planktonic development. The lowest larval abundance was recorded in the period of northern cold wind (January–March) when the fluvial discharges decreased. Bray-Curtis index defined two groups of stations, corresponding to each of the lagoon inlets, persisting throughout the year. The first one, ‘ El Carmen ’, was characterized by larvae of Engraulidae and Gobiidae; estuarine-dependent inhabitants. This group could be considered as a functional extension of the Lagoon to the sea. The second group, ‘ Puerto Real ’, was characterized by highest larval abundance of marine dwellers (e.g. Opisthonema oglinumand Harengula jaguana). This situation suggests that the Puerto Real Inlet could be the main entrance of marine fishes into the Lagoon. These results indicate that the coastal zone off Terminos Lagoon constitutes an important nursery area both for species spending part of their life cycle linked to this estuarine system, and for marine species that migrate towards the Lagoon, carried by local currents.

Brooding and Larval Developmental Periods of the Callianassid Ghost Shrimp, Callianassa Japonica (Decapoda: Thalassinidea)

Estimations of the brooding and larval developmental periods of the callianassid ghost shrimp, Callianassa japonicu Ortmann (Decapoda: Thalassinidea), under field conditions, were made on an intertidal sandflat in western Kyushu, Japan, during the breeding seasons (June-October) of 1992–1994. Females bearing eyed embryos were able to deposit a further batch of eggs 1–5 d after releasing larvae, and it is concluded that such egg deposition was preceded immediately by mating. Based on rearing experiments carried out in the field, the brooding period was found to occupy between 13 and 22 d during June to October, at pore water temperatures of 20.5-27.5°C and salinities of 33.0–34–5‰, a shorter period than those reported for seven other callianassid species. Based on temporal changes in the densities of larvae released by females and newly-settled juveniles on the sandflat, time-lags between the release and settlement events suggested the period of planktonic development with six zoeal stages and the 1st post-larval stage to range from 20 to 25 d during mid-June to mid-August, at water temperatures of 18.5-24.5°C and salinities of 32.0–35.0‰. This is the second shortest larval period among those reported for seven other callianassid species that have five or six zoeal stages.

Changes in Larval Morphology in the Evolution of Benthic Development by Patiriella exigua (Asteroidea: Asterinidae), a Comparison with the Larvae of Patiriella Species with Planktonic Development.

Patiriella exigua (Lamarck) is a small asterinid sea star that deposits large eggs onto the substratum. Development is lecithotrophic and entirely benthic, proceeding without parental care. The embryos develop to the brachiolaria stage before hatching and there is no trace of a bipinnaria larva. In its early stage the larva of P. exigua resembles a planktonic brachiolaria in having one long central brachium and two short lateral brachia. By hatching, the brachia are equal in length, giving the larva a tripod-like appearance. Comparison of the larva of P. exigua with the brachiolaria of Patiriella species with planktonic development supports the hypothesis that the tripod larval form results from differential growth of the lateral brachia. At hatching, the P. exigua larva has a well-developed attachment complex composed of a large adhesive disk and three muscular brachia; the latter bear a striking resemblance to adult tube feet. This hypertrophic elaboration of the brachiolar complex is an adaptation for permanent benthic attachment. Internally, one large enterocoel forms at the anterior end of the archenteron. The archenteron then closes to form the rudiment for the adult gut. Metamorphosis involves gradual resorption of the brachiolar complex concomitant with formation of the first tube feet. The adhesive disk plays a major role in attachment during late metamorphosis but is gradually reduced to a plug of tissue as the tube feet become functional. Juvenile P. exigua are negatively geotactic and float on the water surface, behavior that may act as a mechanism for dispersal. The similarity of the early larva of P. exigua to planktonic brachiolariae suggests that the evolution of benthic lecithotrophy by this species involved modification of a planktonic larval form. These modifications include elimination and reduction of larval feeding structures, formation of one rather than three enterocoels, and hypertrophy of the brachiolar complex to form a tripod larva. Heterochronies in the ontogeny of P. exigua include the delay in hatching to the brachiolaria stage and the accelerated development of the juvenile form and adult skeleton.

Predator-prey relationships within the pelagic community of Neusiedler See

Neusiedler See, a shallow alkaline lake, has become increasingly eutrophic; this enrichment improved the nutritive situation of the herbivorous zooplankton leading to a higher standing stock. A multiple regression analysis of the long-term development of the crustacean plankton indicates that abiotic factors (i.e. wind, temperature) have the most important impact on the community in spring and autumn, biotic factors (i.e. food, predation) during the summer months. Currently an invertebrate (Leptodora kindti) and two planktivorous fish (Pelecus cultratus and Alburnus alburnus) control the population development of Diaphanosoma mongolianum during summer. L. kindti predation acts on immature stages, whereas the fishes consume adult stages. L. kindti densities of 100 to 200 ind. × m-3 affect the prey population to a negligible extent; densities between 300 and 500 ind. × m-3 result in elimination rates of 5% to >40% of the juvenile Diaphanosoma stock. The impacts by the invertebrate predator are pronounced but short-term events. Juvenile and underyearling fish eliminate 4–13% of the zooplankton in the open lake and 5–33% inshore; their predation pressure acts on all younger stages of the crustaceans. Planktivorous fish older than 0 + concentrate on the large food items (adult stages of the crustaceans). A. alburnus is able to consume 6–16% of the zooplankton standing stock during summer. P. cultratus eliminates about 1–49% of D. mongolianum, 1–4% of A. spinosus and 1–31% of L. kindti. From May until August the position of the dominant predator changes, beginning with juvenile fish which are then followed by P. cultratus and then by A. alburnus. Predation by L. kindti is of importance in July and August; at that time it causes suppression of the juveniles of D. mongolianum, but, on the other hand, the older stages of L. kindti are endangered themselves by the predatory impact of planktivorous fish.

Importance of ciliated protozoa in relation to the bacterial and phytoplanktonic biomass in an oligo-mesotrophic lake, during the spring diatom bloom

The abundance and the biomass of bacterial, phytoplanktonic, and ciliate communities were estimated at different depths during the spring planktonic development in an oligo-mesotrophic lake (the Pavin lake).

Microbial decomposition of polymer organic matter related to plankton development in a reservoir: activity of α-, β-glucosidase, and β-N-acetylglucosaminidase and uptake of N-acetylglucosamine

Microbial decomposition of polymer organic matter related to plankton development in a reservoir: activity of α-, β-glucosidase, and β-N-acetylglucosaminidase and uptake of N-acetylglucosamine

Planktonic food chains of a highly humic lake

The development and metabolism of epilimnetic plankton from a highly humic lake was followed in late summer, when the predominant zooplankton species, Daphnia longispina, was very abundant (ca. 200 ind. l -1 ). The experiment was made in two tanks: one with an unaltered plankton assemblage and one with larger zooplankton removed. The scarce phytoplankton community was also simple, consisting mainly of one Cryptomonas two Mallomonas species. The abundance and species composition of smaller plankton was heavily influenced by grazing of Daphnia

A study on pollution of the Puck Lagoon and possibility of restoring the lagoon′s original ecological state

The main aim of the study was to identify mechanisms causing degradation of the Puck Lagoon&prime;s primary environment and to suggest some remedial measures and techniques for its restoration, apart from the newly installed sewage treatment plant near Puck. Results of the study demonstrate an excessive seasonal development of filamentous brown algae of the family Ectocarpaceae. This excessive growth on the bottom of the lagoon plays a significant role in eliminating nutrients from the water, limiting the vernal development of plankton, and considerably increasing water transparency, but strongly affecting living conditions of the original benthic macrophytes. In late summer, the brown algae biomass undergoes rapid decomposition and enriches the sediment with organic matter. Sediment analyses revealed no excessive concentration of heavy metals or organic contaminants, their concentrations being usually proportional to the organic matter content. Experimental seeding- of vascular plants and observations on assemblages of sessile fauna and flora indicate that reintroduction of original species and deployment of artificial reefs is feasible and purposeful.

Phases in the development of riverine plankton: Examples from the rivers Rhine and Meuse

Observations on phyto and zooplankton in two hydrographically different rivers were compared in order to discriminate phases in plankton development. Along the longitudinal axis of the River Rhine a gradual increase in the development of phytoplankton was observed, which reached its maximumca. 100 km before the river flows into its artificial sedimentation area. The development of rotifer populations was slightly retarded as compared with that of phytoplankton and highest population densities were only reached in the sedimentation area. Crustaceans developed in significant numbers, not until the river water had entered the sedimentation area. Development of zooplankton coincided here with a strong decrease in the density of phytoplankton. A similar trend in plankton development was observed in the River Meuse, although in this river the highest densities of phyto and zooplankton already occurred in its middle reaches. The differences in the timing of plankton growth in the two rivers are probably caused by differences in flow regime between both rivers. The River Rhine, which is fed by rainwater and melting of glaciers in the Alps, has a relatively constant discharge. On the other hand, the low discharge of the rain-fed River Meuse combined with an increased residence time of the water as a consequence of large numbers of weirs, allows a full cycle of plankton development long before its discharge into the sea. This phenomenon was also reflected in the silicate cycle in the Meuse, where the consumption by planktonic diatoms and the regeneration of silicate of deposits seem to be important. In contrast, in the main branches of the River Rhine only the effects of silicate consumption were detectable.

Zooplankton distribution in relation to turbidity and related environmental gradients in a large subtropical reservoir: patterns and implications

SUMMARY. A marked gradient in water transparency along the 75 km longitudinal axis of Lake le Roux, a fjord‐like reservoir on the Orange River, South Africa, was consistently evident over several years. On average, Secchi depth transparency doubled from approximately 18 cm at the turbid upper end (which is fed by sediment‐laden Orange River water discharged from Lake Verwoerd), to 33 cm near the dam wall. A variety of changes in abiotic limn logical attributes (temperature, light attenuation, suspended solids and nutrient levels, etc.), and in the abundance and composition of planktonic biota accompany, and may be imposed by, this turbidity gradient. On average, phytoplankton abundance (assessed as chlorophyll content), and total crustacean zooplankton biomass (per unit volume) were lowest at the very top of the lake, but reached maximal values within 15 km ‘downstream’, reflecting the rapid development of plankton. These variables subsequently declined downstream, in contrast to the apparent improvement in conditions for planktonic existence (increasing water transparency, longer water residence time, etc.). The composition of crustacean zooplankton varied along the length of the reservoir. Both seasonal and inter‐annual differences were evident. On average, however, cladocerans (Daphnia, Moina), despite their r‐selected attributes, were especially sparse in the more turbid, uppermost reaches, where advective effects of river inflows were most pronounced. The large predatory copepod Lovenula was surprisingly most abundant in this region, where potential planktonic curstacean prey was scarce. The attainment of maximal zooplankton standing slocks some 15 km downstream was attributable to the proliferation particularly of the herbivorous copepod Metadiaptomus, along with minor increases in several ctadoceran components. The latter continued to increase downstream, in line with declining turbidity, although Daphnia and Moina reached their respective abundance maxima in central and lower reaches of the reservoir, reflecting slight disparities in response to turbidity or related gradients. By contrast.

Ontogcnetic vertical migration in Sebastolobus altivelis as a mechanism for transport of particulate organic matter at continental slope depths

Ontogenetic vertical migration by a demersal fish inhabiting the continental slope off central California was studied as a mechanism for coupling transport of particulate matter between surface waters and the bathyal sea floor. We conducted a 1‐yr survey of Sebastolobus altivelis, a dominant demersal fish, over much of its depth range (400–1,600 m), with a camera sled and bottom trawl. This scorpaenid spawns gelatinous masses of eggs which float to the surface. After hatching, S. altivelis has a protracted pelagic phase and settles as a benthic juvenile at a length of ∼55 mm. We determined the production of particulate organic matter (POM) in masses of eggs as a measure of the passive upward flux of POM, and the recruitment of POM by settling juveniles as a measure of active transport back to the sea floor. Upward and downward fluxes of particulate organic carbon (POC) associated with planktonic development and ontogenetic migration of S. altivelis were 4.1–5.8 and 3.0–5.5 mg C m−2 yr−1. These upward and downward fluxes are lower by factors of ∼10 and ∼104 than the reported passive upward and downward fluxes measured with particulate interceptor traps in the eastern North Pacific. These data suggest that at bathyal depths, it is unlikely that vertical ontogenetic migrations contribute &gt;1% to the POC flux reaching the deep‐sea floor. The upward flux of POC as buoyant eggs could represent, however, a significant portion of the total upward flux.

Phases in the development of riverine plankton: Examples from the rivers Rhine and Meuse

Observations on phyto and zooplankton in two hydrographically different rivers were compared in order to discriminate phases in plankton development. Along the longitudinal axis of the River Rhine a gradual increase in the development of phytoplankton was observed, which reached its maximumca. 100 km before the river flows into its artificial sedimentation area. The development of rotifer populations was slightly retarded as compared with that of phytoplankton and highest population densities were only reached in the sedimentation area. Crustaceans developed in significant numbers, not until the river water had entered the sedimentation area. Development of zooplankton coincided here with a strong decrease in the density of phytoplankton. A similar trend in plankton development was observed in the River Meuse, although in this river the highest densities of phyto and zooplankton already occurred in its middle reaches. The differences in the timing of plankton growth in the two rivers are probably caused by differences in flow regime between both rivers. The River Rhine, which is fed by rainwater and melting of glaciers in the Alps, has a relatively constant discharge. On the other hand, the low discharge of the rain-fed River Meuse combined with an increased residence time of the water as a consequence of large numbers of weirs, allows a full cycle of plankton development long before its discharge into the sea. This phenomenon was also reflected in the silicate cycle in the Meuse, where the consumption by planktonic diatoms and the regeneration of silicate of deposits seem to be important. In contrast, in the main branches of the River Rhine only the effects of silicate consumption were detectable.

Relationships of food uptake and body components of Calanus finmarchicus, C. glacialis and C. hyperboreus to particulate matter and water characteristics in Fram Strait

During MIZEX'83 and MIZEX'84 food composition and food uptake by three Calanus species were investigated in the northern Greenland Sea. Samples were obtained from open water, the marginal ice zone, the pack ice region and the East Greenland shelf polynya. Food uptake of Calanus spp. was determined in ship-board incubation experiments under in situ conditions and volume of particulate matter in the upper 300 m of the water column was measured. Principal components analysis was used for data evaluation. Under the pack ice strongly reduced amounts of particulate matter in the euphotic zone were found while concentrations were elevated in the marginal ice zone and the East Greenland shelf polynya. Food uptake of C. finmarchicus was correlated to food concentration. In the pack ice region ingestion was close to zero for all species investigated. Likewise body weights were significantly lower than in MIZ or polynya. Principal components analysis of physical data shows a clustering of MIZ station groups at high temperature and salinity, clearly separated from pack ice and polynya stations. Analysis of biological data results in the same grouping of stations demonstrating a strong influence of hydrographic conditions on plankton development. It is concluded that herbivorous copepods cannot sustain themselves in the pack ice region but only at places of high productivity like marginal ice zone and East Greenland shelf polynya.

Phosphorus transformations in the epilimnion of humic lakes: biological uptake of phosphate

SUMMARY. 1. The hypothesis that dissolved humic material (DHM) stimulates bacterial involvement in phosphorus transformations and may thus lead to decreased accessibility of phosphorus to algae was investigated by studying three small forest lakes in southern Finland representing a wide range of concentrations of DHM.2. Other chemical differences between the three lakes were slight, although the most humic lake exhibited higher concentrations of total phosphorus and of molybdate‐reactive phosphorus. Bacterial biomass did not differ significantly between the lakes, but algal biomass was significantly lower at higher DHM concentrations. Consequently the ratio of algal biomass to bacterial biomass was significantly lower in the most humic lake.3. Uptake of phosphorus from added 33PO4 was partitioned between algal and bacterial size fractions by differential filtration. No significant variation between lakes was found in the proportion of particulate 33P recovered from the algal fraction.4. Turnover times for phosphate were significantly longer in the most humic lake and also showed lower variability. In general turnover times were long in comparison with values reported from many other lakes. Only briefly in mid summmer did turnover times in two of the lakes shorten to values which would indicate that demand for phosphate was outstripping supply.5. Short‐term storage of samples from the most humic lake stimulated biological incorporation of 33P, but additions of nitrogen and iron had little effect on phosphate uptake.6. In these small forest lakes it is probable that no single nutrient consistently limits plankton development. Since no evidence was found that DHM shifts the balance of plankton phosphate uptake away from algae towards bacteria, the influence of DHM on phosphorus transformations may rather be through chemical regulation of free phosphate availability.