Density-dependent Changes Research Articles

Breeding skuas in Orkney: a 2010 census indicates density-dependent population change driven by both food supply and predation

Great Skuas Stercorarius skua and Arctic Skuas S. parasiticus were counted on their breeding grounds in Orkney during June 2010. Great Skua numbers had fallen by 23% overall since the previous census in 2000. However, the decline was not uniform across Orkney, the major colony on Hoy declining by 32% but with many smaller colonies actually increasing. Possible reasons for this difference are discussed. Arctic Skua numbers fell by 47% in the same time period and by 64% from their peak in 1992. We found evidence of intra-specific negative density dependence in trends of both skua species, suggesting that declines may have been driven by low food availability. In addition, we found a negative relationship between Arctic Skua trends between 2000 and 2010 and the numbers of Great Skuas present in each colony in 2000. These findings suggest that whilst scarcity of food may have contributed to Arctic Skua declines, predation by Great Skuas is likely to have played an additional role. Predation on Arctic Skuas, both adults and young, is also likely to be ultimately attributable to a scarcity of alternative fish prey for Great Skuas.

Cortical Neural Precursors Inhibit Their Own Differentiation via N-Cadherin Maintenance of β-Catenin Signaling

Little is known about the architecture of cellular microenvironments that support stem and precursor cells during tissue development. Although adult stem cell niches are organized by specialized supporting cells, in the developing cerebral cortex, neural stem/precursor cells reside in a neurogenic niche lacking distinct supporting cells. Here, we find that neural precursors themselves comprise the niche and regulate their own development. Precursor-precursor contact regulates beta-catenin signaling and cell fate. In vivo knockdown of N-cadherin reduces beta-catenin signaling, migration from the niche, and neuronal differentiation in vivo. N-cadherin engagement activates beta-catenin signaling via Akt, suggesting a mechanism through which cells in tissues can regulate their development. These results suggest that neural precursor cell interactions can generate a self-supportive niche to regulate their own number.

Effect of non‐random dispersal strategies on spatial coexistence mechanisms

1. Random dispersal leads to spatial coexistence via two mechanisms (emigration-mediated and source-sink), both of which involve the movement of organisms from areas of higher to lower fitness. What is not known is whether such coexistence would occur if organisms dispersed non-randomly, using cues such as density and habitat quality to gauge fitness differences between habitats. Here, I conduct a comparative analysis of random and non-random dispersal strategies in a foodweb with a basal resource, top predator, and two intermediate consumers that exhibit a trade-off between competitive ability and predator susceptibility. 2. I find a striking contrast between density- and habitat-dependent dispersal in their effects on spatial coexistence. Dispersal in response to competitor and predator density facilitates coexistence while dispersal in response to habitat quality (resource productivity and predator pressure) inhibits it. Moreover, density-dependent dispersal changes species' distribution patterns from interspecific segregation to interspecific aggregation, while habitat-dependent dispersal preserves the interspecific segregation observed in the absence of dispersal. Under density-dependent dispersal, widespread spatial coexistence results in an overall decline in the abundance of the inferior competitor that is less susceptible to predation and an overall increase in the abundance of the superior competitor that is more susceptible to predation. Under habitat-dependent dispersal, restricted spatial coexistence results in species' abundances being essentially unchanged from those observed in the absence of dispersal. 3. A key outcome is that when the superior competitor moves in the direction of increasing fitness but the inferior competitor does not, spatial coexistence is possible in both resource-poor and resource-rich habitats. However, when the inferior competitor moves in the direction of increasing fitness but the superior competitor does not, spatial coexistence is precluded in resource-poor habitats and greatly reduced in resource-rich habitats. This suggests that species-specific differences may play an important role in driving spatial coexistence patterns. 4. The comparative framework yields predictions that can be tested with experiments that manipulate the relative mobilities of interacting species, or observational data on relative abundances and distribution patterns.

Cell density-dependent regulation of p73 in breast cancer cells

Molecular regulation of p73, a p53 family member, remains unclear. Here we report that p73 expression is significantly regulated by cell densities. In particular, we found that p73alpha and p73beta are differentially regulated. While p73beta protein levels were inversely correlated with cell densities, p73alpha protein levels behaved oppositely. We further showed that density-dependent changes of p73alpha follow the same patterns as E2F-1 and TAp73 mRNA levels, suggesting transcriptional regulation. Our data also suggest that high levels of p73beta at lower densities may be due to increased protein stability. However, AIP-4/Itch appeared not to be involved in downregulation of p73beta at high densities. Moreover, we also found that subcellular location of p73 isoforms changes with the culture density increases. While high level of p73beta at low density was mainly presented in the nucleus, low levels of this protein at high densities were mainly in the cytosol. Taken together, these findings reveal a novel mechanism that differentially regulates p73 isoforms and underscores the role of cell-cell interaction in p73 regulation, which may advance our understanding of p73 expression and function in human cancers.

Memoirs of a locust: Density-dependent behavioral change as a model for learning and memory

A locust outbreak is a stupendous natural phenomenon that remains in the memory of whoever has been lucky (or unlucky) enough to witness it. Recent years have provided novel and important insights into the neurobiology of locust swarming. However, the central nervous system processes that accompany and perhaps even lie at the basis of locust phase transformation are still far from being fully understood. Our current work deals with the memory of a locust outbreak from a new perspective: that of the individual locust. We take locust density-dependent phase transformation – a unique example of extreme behavioral plasticity, and place it within the context of the accepted scheme of learning and memory. We confirm that a short time period of exposure to a small crowd of locusts is sufficient to induce a significant behavioral change in a previously solitary locust. Our results suggest that part of the behavioral change is due to long-term habituation of evasive and escape responses. We further demonstrate that the memory of a crowding event lasts for at least 24 h, and that this memory is sensitive to a protein synthesis blocker. These findings add much to our understanding of locust density-dependent phase polyphenism. Furthermore, they offer a novel and tractable model for the study of learning and memory-related processes in a very distinctive behavioral context.

Is there direct and delayed density dependent variation in population structure in a temperate European cyclic vole population?

Population structure, in terms of the body mass, condition, sex and reproductive status of individuals, has been found to vary in phase with population density in cyclic populations of microtine rodents. Because sustained population cycles involve delayed density dependent changes in the population growth rate, we would expect at least some life history traits also to depend on past densities. Detailed, long‐term studies of changes in vole life history traits are however few, and are largely restricted to northern Europe. In view of the uncertainty as to whether the cyclic microtine populations of western Europe represent the same phenomenon as those of northern Europe, we studied temporal variation in the structure of a clearly cyclic population of the common vole Microtus arvalis Pallas, in the cereal plains of mid‐western France. Our data set contains seasonal, individual‐level data from long‐term, large‐scale trapping covering four entire population cycles. We found considerable cyclic variation in population structure in spring (April), but less so in summer (June). In spring of post‐peak years, animals were of low body weight and body condition (particularly females), litter sizes were smaller and there was a reduction in the proportion of breeders. All of these could be proximal drivers of increased mortality rates, or decreased birth rates, contributing to the population declines. Few life history traits, however, showed direct density dependent variation, and none of the traits studied here showed delayed density dependence. We have shown declines in the fecundity and body condition of voles from a western European population that coincides with, and may be a proximal cause of, cyclic declines in population density. Closer attention to proximal causes, by which ecological processes drive cycles, could clarify the extent to which microtine cycles across Europe represent a single phenomenon.

Optimal sample size for composite sampling with subsampling, when estimating the proportion of pecky rice grains in a field

The proportion of pecky rice grains has been estimated empirically using composite sampling with subsampling. The procedure can be summarized as follows: (1) A fixed number of rice plants, n 1, are drawn at random in the paddy field; (2) all of the rice grains in the collected rice plants are mixed well to form a composite; (3) a portion of the grains, n 2, is drawn at random from the composite; and (4) the collected grains are examined by eye to estimate the proportion of pecky rice grains. We propose a method for determining the optimal sample size in estimating the proportion of defective items by this type of composite sampling with subsampling. Spatial heterogeneity in the proportion of defective items is included in the estimation. We use Taylor’s power law to describe the density-dependent change of spatial heterogeneity. In controlling the precision of the estimate, we use the relative precision, D, which is defined by the coefficient of variation of the estimated proportion. We propose a rejection procedure in which the product is rejected if the estimate of proportion with D=0.25 is larger than a predetermined tolerable threshold of proportion. We also consider another control criterion in which the consumer’s risk, \, is controlled by a zero-tolerance method. Finally, we examine the relationship between the two control criteria. Numerical examples of calculations are available in the online supplements.

Density‐dependent life‐history compensation of an iteroparous salmonid

Over the course of a decade, the bull trout (Salvelinus confluentus) population in Lower Kananaskis Lake, Alberta, Canada, recovered from a heavily overexploited state, experiencing a 28-fold increase in adult abundance after the implementation of zero-harvest regulations. This system provided a unique opportunity to monitor the changes in life-history characteristics in a natural population throughout the recovery process. The purpose of this study was to examine the degree to which life-history traits were able to compensate for harvest-induced changes and the implications of this for management. Density-dependent changes in growth, survival, and reproductive life-history characteristics were observed. As density increased, maturation was delayed, and the frequency of skipped reproductive events, primarily by individuals of poor condition, increased. However, size at maturation and the proportion of fish skipping reproduction differed between the sexes, suggesting that life-history trade-offs differ between the sexes. The rapid response of these life-history traits to changes in density suggests that these changes were primarily due to phenotypic plasticity, although the importance of natural and artificial selection should not be discounted. The magnitude of the variation in the traits represents the degree to which the population was able to compensate for overharvest, although the overexploited state of the population at the beginning of the study demonstrates it was not able to fully compensate for this mortality. However, no evidence of depensatory processes was found. This, in combination with the plasticity of the life-history traits, has important implications for the resilience of the population to overharvest. Furthermore, density-dependent growth may have the unintended result of making size-based regulations less conservative at low levels of population abundance, as younger fish, perhaps even immature fish, become vulnerable to harvest. Finally, the variation in life-history traits in relation to evolutionary change is discussed. Results from this study demonstrate the importance of considering not only survival, but also changes in life-history characteristics for management and conservation.

Reconstruction of the P2X2 Receptor Reveals a Vase-Shaped Structure with Lateral Tunnels above the Membrane

In response to the intercellular messenger ATP, P2X receptors transfer various sensory information, including pain. Here we have reconstructed the structure of the P2X(2) receptor at 15 A resolution from more than 90,000 particle images, taken with a cryo-electron microscope equipped with a helium-cooled stage. This three-dimensional depiction, presumably in a closed state, revealed an elongated vase-shaped structure 202 A in height and 160 A in major diameter. The extracellular and transmembrane domains present a two-layered structure, in which a sparse outer layer surrounds a pore-forming inner density. The decreased diameter of a putative ion-conducting pathway at the middle of the membrane was considered to be the narrowest part of the pore, which has been predicted from electrophysiological studies. The sparse, extended structure of the P2X(2) receptor indicates a loose assembly of subunits, which could be a basis for the activation-dependent pore dilation of P2X receptors.

Parasite manipulation of brain monoamines in California killifish ( Fundulus parvipinnis ) by the trematode Euhaplorchis californiensis

California killifish (Fundulus parvipinnis) infected with the brain-encysting trematode Euhaplorchis californiensis display conspicuous swimming behaviours rendering them more susceptible to predation by avian final hosts. Heavily infected killifish grow and reproduce normally, despite having thousands of cysts inside their braincases. This suggests that E. californiensis affects only specific locomotory behaviours. We hypothesised that changes in the serotonin and dopamine metabolism, essential for controlling locomotion and arousal may underlie this behaviour modification. We employed micropunch dissection and HPLC to analyse monoamine and monoamine metabolite concentrations in the brain regions of uninfected and experimentally infected fish. The parasites exerted density-dependent changes in monoaminergic activity distinct from those exhibited by fish subjected to stress. Specifically, E. californiensis inhibited a normally occurring, stress-induced elevation of serotonergic metabolism in the raphae nuclei. This effect was particularly evident in the experimentally infected fish, whose low-density infections were concentrated on the brainstem. Furthermore, high E. californiensis density was associated with increased dopaminergic activity in the hypothalamus and decreased serotonergic activity in the hippocampus. In conclusion, the altered monoaminergic metabolism may explain behavioural differences leading to increased predation of the infected killifish by their final host predators.

Effectiveness of European Atlanto-Mediterranean MPAs: Do they accomplish the expected effects on populations, communities and ecosystems?

The success of MPAs in conserving fishing resources and protecting marine biodiversity relies strongly on how well they meet their planned (or implicit) management goals. From a review of empirical studies aiming at assessing the ecological effects of Mediterranean and Macaronesian MPAs, we conclude that establishing an MPA is successful for (i) increasing the abundance/biomass, (ii) increasing the proportion of larger/older individuals, and (iii) enhancing the fecundity of commercially harvested populations; also, MPAs demonstrated to be effective for (iv) augmenting local fishery yields through biomass exportation from the protected area, and (v) inducing shifts in fish assemblage structure by increasing the dominance of large predator species. However, the attraction for tourism and diving due to ecological benefits of protection can cause damages likely to reverse some of the MPA effects. Other expected effects are more subject to uncertainty, and hence need more research, such as (vi) causing density-dependent changes in life history traits and (vii) protecting the recruitment of commercially important species, (viii) protecting marine biodiversity (including genetic diversity), (ix) causing ecosystem-wide effects such as trophic cascades, and (x) increasing community and ecosystem stability, thus promoting resilience and faster recovery from disturbance. Meta-analysis of data arising from these case studies are used to establish the overall effect of MPAs, and its relationship to MPA features, such as size of no-take area or time since protection. Based on the review and the meta-analyses, specific recommendations are provided for MPA management, regarding the establishment of goals and objectives, site selection, MPA design and zoning, planning, and monitoring. Finally, a series of recommendations for MPA research are offered to drive future research in MPA issues in the Mediterranean and Macaronesia.

Do North Sea cod (Gadus morhua) fisheries maintain high catch rates at low stock size?

This study presents an investigation of the relationship between stock size of North Sea cod ( Gadus morhua ) and catch rates in seven commercial fishing fleets. The shape of the relationship was estimated using a new model allowing both density-dependent changes in catchability and bias in the assessment biomass estimates. Catchability in fisheries targeting a mixed species composition either remained constant or decreased with decreasing stock size, whereas catchability in targeted cod fisheries increased with decreasing stock size. However, even in the cases where catchability increased, the change was insufficient to compensate for the decrease in stock size, and catch rates of all fleets decreased. Two factors that could lead to nonconstant catchability were investigated: (i) the presence of a decoupling between stock size and density in high-density areas and (ii) the presence of concurrent shifts in the spatial distribution of the cod stock and the cod fishery. No evidence of the former was found, but there was a northern shift in the spatial distribution of both effort and the cod stock.

Temporal changes in kin structure through a population cycle in a territorial bird, the red grouse Lagopus lagopus scoticus

Populations of red grouse (Lagopus lagopus scoticus) undergo regular multiannual cycles in abundance. The 'kinship hypothesis' posits that such cycles are caused by changes in kin structure among territorial males producing delayed density-dependent changes in aggressiveness, which in turn influence recruitment and regulate density. The kinship hypothesis makes several specific predictions about the levels of kinship, aggressiveness and recruitment through a population cycle: (i) kin structure will build up during the increase phase of a cycle, but break down prior to peak density; (ii) kin structure influences aggressiveness, such that there will be a negative relationship between kinship and aggressiveness over the years; (iii) as aggressiveness regulates recruitment and density, there will be a negative relationship between aggressiveness in one year and both recruitment and density in the next; (iv) as kin structure influences recruitment via an affect on aggressiveness, there will be a positive relationship between kinship in one year and recruitment the next. Here we test these predictions through the course of an 8-year cycle in a natural population of red grouse in northeast Scotland, using microsatellite DNA markers to resolve changing patterns of kin structure, and supra-orbital comb height of grouse as an index of aggressiveness. Both kin structure and aggressiveness were dynamic through the course of the cycle, and changing patterns were entirely consistent with the expectations of the kinship hypothesis. Results are discussed in relation to potential drivers of population regulation and implications of dynamic kin structure for population genetics.

Genetic variation for parental effects on the propensity to gregarise in Locusta migratoria

BackgroundEnvironmental parental effects can have important ecological and evolutionary consequences, yet little is known about genetic variation among populations in the plastic responses of offspring phenotypes to parental environmental conditions. This type of variation may lead to rapid phenotypic divergence among populations and facilitate speciation. With respect to density-dependent phenotypic plasticity, locust species (Orthoptera: family Acrididae), exhibit spectacular developmental and behavioural shifts in response to population density, called phase change. Given the significance of phase change in locust outbreaks and control, its triggering processes have been widely investigated. Whereas crowding within the lifetime of both offspring and parents has emerged as a primary causal factor of phase change, less is known about intraspecific genetic variation in the expression of phase change, and in particular in response to the parental environment. We conducted a laboratory experiment that explicitly controlled for the environmental effects of parental rearing density. This design enabled us to compare the parental effects on offspring expression of phase-related traits between two naturally-occurring, genetically distinct populations of Locusta migratoria that differed in their historical patterns of high population density outbreak events.ResultsWe found that locusts from a historically outbreaking population of L. migratoria expressed parentally-inherited density-dependent phase changes to a greater degree than those from a historically non-outbreaking population.ConclusionBecause locusts from both populations were raised in a common environment during our experiment, a genetically-based process must be responsible for the observed variation in the propensity to express phase change. This result emphasizes the importance of genetic factors in the expression of phase traits and calls for further investigations on density-dependent parental effects in locust phase change. More population samples with different outbreak histories need to be analyzed to demonstrate that differences in propensity to gregarise evolve because of different outbreak histories.

Egg Thiamine Status of Lake Ontario Salmonines 1995–2004 with Emphasis on Lake Trout

Alewives ( Alosa pseudoharengus), the major prey fish for Lake Ontario, contain thiaminase. They are associated with development of a thiamine deficiency in salmonines which greatly increases the potential for developing an early mortality syndrome (EMS). To assess the possible effects of thiamine deficiency on salmonine reproduction we measured egg thiamine concentrations for five species of Lake Ontario salmonines. From this we estimated the proportion of families susceptible to EMS based on whether they were below the ED20, the egg thiamine concentration associated with 20% mortality due to EMS. The ED20s were 1.52, 2.63, and 2.99 nmol/g egg for Chinook salmon ( Oncorhynchus tshawytscha), lake trout ( Salvelinus namaycush), and coho salmon ( Oncorhynchus kisutch), respectively. Based on the proportion of fish having egg thiamine concentrations falling below the ED20, the risk of developing EMS in Lake Ontario was highest for lake trout, followed by coho ( O. kisutch), and Chinook salmon, with the least risk for rainbow trout ( O. mykiss). For lake trout from western Lake Ontario, mean egg thiamine concentration showed significant annual variability during 1994 to 2003, when the proportion of lake trout at risk of developing EMS based on ED20 ranged between 77 and 100%. Variation in the annual mean egg thiamine concentration for western Lake Ontario lake trout was positively related (p < 0.001, r 2 = 0.94) with indices of annual adult alewife biomass. While suggesting the possible involvement of density-dependent changes in alewives, the changes are small relative to egg thiamine concentrations when alewife are not part of the diet and are of insufficient magnitude to allow for natural reproduction by lake trout.

CAN WE MEASURE CARRYING CAPACITY WITH FORAGING BEHAVIOR?

Carrying capacity is one of the most important, yet least understood and rarely estimated, parameters in population management and modeling. A simple behavioral metric of carrying capacity would advance theory, conservation, and management of biological populations. Such a metric should be possible because behavior is finely attuned to variation in environment including population density. We connect optimal foraging theory with population dynamics and life history to develop a simple model that predicts this sort of adaptive density-dependent change in food consumption. We then confirm the model's unexpected and manifold predictions with field experiments. The theory predicts reproductive thresholds that alter the marginal value of energy as well as the value of time. Both effects cause a pronounced discontinuity in quitting-harvest rate that we revealed with foraging experiments. Red-backed voles maintained across a range of high densities foraged at a lower density-dependent rate than the same animals exposed to low-density treatments. The change in harvest rate is diagnostic of populations that exceed their carrying capacity. Ecologists, conservation biologists, and wildlife managers may thus be able to use simple and efficient foraging experiments to estimate carrying capacity and habitat quality.

Cell culture density dependent toxicity and chromatin changes upon cadmium treatment in murine pre-B-cells

Murine pre-B-cells grown in the presence of lower (1 microM) or higher (5 microM) concentration of cadmium chloride were separated into 13 fractions by centrifugal elutriation. The rate of DNA synthesis after cadmium treatment determined in permeable cells was dependent on cell culture density during cadmium treatment. Cell cycle analysis revealed a shift in the profile of DNA synthesis from replicative to repair DNA synthesis upon cadmium treatment. The study of the relationship between cell culture density and cell diameter at lower and higher cell densities in the presence of 1 microM cadmium chloride concentration showed that a. at 5 x 10(5) cell/ml or lower densities cells were shrinking indicating apoptotic changes, b. at higher cell culture densities the average cell size increased, c. the treatment of cells with low CdCl(2) concentration (1 microM) at higher cell culture density (>5 x 10(5) cell/ml) did not change significantly the average cell diameter. At 5 microM cadmium concentration and higher cell culture densities (>5 x 10(5) cell/ml) the average cell size decreased in each elutriated fraction. Most significant inhibition of cell growth took place in early S phase (2.0-2.5 C value). Apoptotic chromatin changes in chromatin structure after cadmium treatment were seen as large extensive disruptions, holes in the nuclear membrane and stickiness of incompletely folded chromosomes.

Changes in baseline growth and maturation parameters of Northwest Atlantic porbeagle, Lamna nasus, following heavy exploitation

We tested for density-dependent changes in growth and maturation of Northwest Atlantic porbeagle (Lamna nasus) after the population declined by 75%–80% from fishing. Vertebrae and reproductive data collected from the virgin (1961–1966) and exploited (1993–2004) populations were analysed to test for differences in growth rate and age and length at maturity between the time periods. We detected significant differences between reparameterized von Bertalanffy growth models for each period, using likelihood ratio tests. Beyond an age of 7 years, mean length at age was greater during 1993–2004 than during 1961–1966. Between 1961–1963 and 1999–2001, length at maturity decreased in males (from 179 to 174 cm curved fork length (CFL)) and was invariant in females (216 cm CFL), whereas age at maturity declined in both males (from 8 to 7 years) and females (from 19 to 14 years). An analysis of porbeagle temperature associations indicated that sharks occupied comparable temperature conditions during the mid-1960s and 1990s, ruling out the possibility of temperature-induced growth changes. The observed increase in growth rate and decrease in age at maturity following exploitation support the hypothesis of a compensatory density-dependent growth response.

Fixation in haploid populations exhibiting density dependence I: The non-neutral case

We extend the one-locus two allele Moran model of fixation in a haploid population to the case where the total size of the population is not fixed. The model is defined as a two-dimensional birth-and-death process for allele number. Changes in allele number occur through density-independent death events and birth events whose per capita rate decreases linearly with the total population density. Uniquely for models of this type, the latter is determined by these same birth-and-death events. This provides a framework for investigating both the effects of fluctuation in total population number through demographic stochasticity, and deterministic density-dependent changes in mean density, on allele fixation. We analyze this model using a combination of asymptotic analytic approximations supported by numerics. We find that for advantageous mutants demographic stochasticity of the resident population does not affect the fixation probability, but that deterministic changes in total density do. In contrast, for deleterious mutants, the fixation probability increases with increasing resident population fluctuation size, but is relatively insensitive to initial density. These phenomena cannot be described by simply using a harmonic mean effective population size.

Transfer of diazotrophic nitrogen to mesozooplankton in Kiel Fjord, Western Baltic Sea: a mesocosm study

In a mesocosm experiment in Kiel Fjord, the plankton community <250 µm was exposed to a mesozooplankton density gradient (5 to 80 individuals l–1) dominated by the calanoid copepod Acartia clausi. Over the experimental period (9 d), the diazotrophic cyanobacterium Nodularia spumigena increased exponentially, irrespective of mesozooplankton densities, attaining maximum concentrations of 1200 cells ml–1 (3700 filaments l–1). At the end of the experiment, the δ15N of particulate organic matter was negatively correlated with N. spumigena concentrations, indicating the fixation of isotopically ‘light’, diazotrophic nitrogen. In all treatments, final copepod δ15N were lower (–0.7 to –2.7‰) than initial copepod δ15N, indicating the transfer of diazotrophic nitrogen to mesozooplankton. Based on a simple isotopic mixing model, diazotrophic nitrogen was calculated to contribute 45 to 6% to final copepod δ15N along the mesozooplankton gradient. This translates to a transfer of 2 to 24% of net nitrogen fixation to the mesozooplankton standing stock. The absence of any mesozooplankton impact on N. spumigena, and the negative impact found for other microplankton, including diatoms and ciliates, suggest that diazotrophically fixed nitrogen reached mesozooplankton indirectly through trophic vectors. This is consistent with the fact that copepod δ15N decreased with decreasing mesozooplankton densities, since only a quantitatively limited dietary source may be expected to result in density-dependent changes in copepod δ15N. Considering that natural mesozooplankton densities in the Baltic Sea rarely exceed 10 ind. l–1, the contribution of diazotrophically fixed nitrogen to mesozooplankton may be substantial (23 to 45%) during summer blooms of diazotrophic cyanobacteria.