Trophic switches in pelagic systems

Ecological studies need experimentation to test concepts and to disentangle causality in community dynamics. While simple models have given substantial insights into population and community dynamics, recent ecological concepts become increasingly complex. The globally important pelagic food web dynamics are well suited to test complex ecological concepts. For instance, trophic switches of individual organisms within pelagic food webs can elongate food webs or shift the balance between autotroph and heterotroph carbon fluxes. Here, we summarize results from mesocosm experiments demonstrating how environmental drivers result in trophic switches of marine phytoplankton and zooplankton communities. Such mesocosm experiments are useful to develop and test complex ecological concepts going beyond trophic level–based analyses, including diversity, individual behavior, and environmental stochasticity

Dynamics of Dissolved and Particulate Polyunsaturated Aldehydes in Mesocosms Inoculated with Different Densities of the Diatom Skeletonema marinoi

A survey of the production of polyunsaturated aldehydes (PUA) of manipulated plankton communities is presented here. PUA are phytoplankton-derived metabolites that are proposed to play an important role in chemically mediated plankton interactions. Blooms of different intensities of the diatom Skeletonema marinoi were generated in eight mesocosms filled with water from the surrounding fjord by adding different amounts of a starting culture and nutrients. This set-up allowed us to follow PUA production of the plankton community over the entire induced bloom development, and to compare it with the natural levels of PUA. We found that S. marinoi is a major source for the particulate PUA 2,4-heptadienal and 2,4-octadienal (defined as PUA released upon wounding of the diatom cells) during the entire bloom development. Just before, and during, the decline of the induced diatom blooms, these PUA were also detected in up to 1 nM concentrations dissolved in the water. In addition, we detected high levels of the PUA 2,4-decadienal that was not produced by the diatom S. marinoi. Particulate decadienal correlated well with the cell counts of the prymnesiophyte Phaeocystis sp. that also developed in the fertilized mesocosms. Particulate decadienal levels were often even higher than those of diatom-derived PUA, indicating that PUA sources other than diatoms should be considered when it comes to the evaluation of the impact of these metabolites

Environmental variability in aquatic ecosystems: Avenues for future multifactorial experiments

The relevance of considering environmental variability for understanding and predicting biological responses to environmental changes has resulted in a recent surge in variability-focused ecological research. However, integration of findings that emerge across studies and identification of remaining knowledge gaps in aquatic ecosystems remain critical. Here, we address these aspects by: (1) summarizing relevant terms of variability research including the components (characteristics) of variability and key interactions when considering multiple environmental factors; (2) identifying conceptual frameworks for understanding the consequences of environmental variability in single and multifactorial scenarios; (3) highlighting challenges for bridging theoretical and experimental studies involving transitioning from simple to more complex scenarios; (4) proposing improved approaches to overcome current mismatches between theoretical predictions and experimental observations; and (5) providing a guide for designing integrated experiments across multiple scales, degrees of control, and complexity in light of their specific strengths and limitations

A global agenda for advancing freshwater biodiversity research

Global freshwater biodiversity is declining dramatically, and meeting the challenges of this crisis requires bold goals and the mobilisation of substantial resources. While the reasons are varied, investments in both research and conservation of freshwater biodiversity lag far behind those in the terrestrial and marine realms. Inspired by a global consultation, we identify 15 pressing priority needs, grouped into five research areas, in an effort to support informed stewardship of freshwater biodiversity. The proposed agenda aims to advance freshwater biodiversity research globally as a critical step in improving coordinated actions towards its sustainable management and conservation

A global agenda for advancing freshwater biodiversity research

Global freshwater biodiversity is declining dramatically, and meeting the challenges of this crisis requires bold goals and the mobilisation of substantial resources. While the reasons are varied, investments in both research and conservation of freshwater biodiversity lag far behind those in the terrestrial and marine realms. Inspired by a global consultation, we identify 15 pressing priority needs, grouped into five research areas, in an effort to support informed stewardship of freshwater biodiversity. The proposed agenda aims to advance freshwater biodiversity research globally as a critical step in improving coordinated actions towards its sustainable management and conservation.Peer reviewe

Predation by copepods upon natural populations of Phaeocystis pouchetii as a function of the physiological state of the prey

Conflicting data have been previously presented on the ability of copepods to prey upon the prymneslophyte Phaeocystis pouchetii. While some have suggested that gelatinous colonies of this species contain biochemical substances that prevent their consumption, others have shown that both single cells and colonies of P. pouchetii can serve as an excellent food source. The present study presents data from feeding experiments using 4 species of copepods and natural samples of phytoplankton prey from a south-north transect during May 1989 in the Barents Sea Natural phytoplankton contained P. pouchetii colonies inn association with varying amounts of diatoms. Along the transect these colonies varied from highly fluorescent and healthy in the north, to weakly fluorescent in the south. Results of experiments using both image analysis and radiotracer techniques indicate that diatoms were actively preyed upon in all experiments, with long-chain-forming species as the preferred food. Predation upon P. pouchetii colonies as dependent upon the physiological condition of the colonies. Healthy colonies were not consumed, while susceptible colonies were consumed at rates 2 to 10 times those for chain-forming diatoms. The selective predation described here has important implications for species composition in Arctic waters

Correcting for underestimation of microzooplankton grazing in bottle incubation experiments with mesozooplankton

Bottle incubation experiments are widely used in mesozooplankton grazing studies. However, we have shown here that traditional particle removal experiments with Calanus finmarchicus and C. helgolandicus as grazers on natural plankton may yield low or even statistically significant (p < 0.05) negative grazing estimates, even though negative grazing rates are impossible. Low grazing rates are often reported, especially on smaller prey types, despite abundant food and significant egg production. Microzooplankton, such as ciliates, show higher biomass-specific grazing rates on algae than do copepods and other mesozooplankton. Instead, copepods often selectively feed on the microzooplankton. Thus, apparent negative rates would be expected when the release of microzooplankton grazing pressure outweighs the copepod grazing rates on the same food items in the incubation bottle. We show that this potentially large bias increases with microzooplankton community grazing pressure in the control. A simplified general method to correct for this bias is presented and compared with the original method (Nejstgaard et al. 1997, Mar Ecol Prog Ser 147:197–217). Although complexity and the need for taxonomic accuracy are reduced in the general method, the results are not significantly different between the 2 methods. Both methods also show a good fit with ingestion rates estimated from faecal pellet production. We suggest that the general method be combined with automated sample treatment in future studies. In addition, we argue that carefully estimated faecal volume production provides a simple and quick overall feeding estimate with important advantages over the common gut pigment technique, and it may be used as an independent method in bottle incubation experiments

Zooplankton growth, diet and reproductive success compared in simultaneous diatom- and flagellatemicrozooplankton-dominated plankton blooms

Journal homepage: http://www.int-res.com/journals/meps/Development of mesozooplankton biomass, feeding activity and reproductive success of the copepod Calanus helgolandicus were compared in blooms of natural plankton in 7 mesocosms on the west coast of Norway between 28 August and 28 September 1996. Nutrient content and turbulence in enclosures of 27 m3 natural sea water were manipulated in situ in order to generate blooms dominated by diatoms and flagellate-microzooplankton respectively. The diatom-dominated mesocosms reached the highest algal biomasses. Calanus helgolandicus generally preferred non-diatom food, such as ciliates and metazoans, and showed similar ingestion rates in both systems. The female body-carbon-specific nauplii production was highest (10.6% d–1) during a flagellate-microzooplankton bloom, before it dropped to ca 1.7% d–1 during a bloom of Dictyocha speculum. In the diatomdominated water it dropped to as low as 0.1%, and was always <5.2% d–1. The total standing stock of calanoid copepods increased about 2.6 times more in the flagellate-microzooplankton-dominated mesocosms. The drop in reproductive success in diatom-dominated water was recorded when feeding rates were high, and ciliates and other prey made up a substantial part of the diet. This suggests that either all prey, including the ciliates, were of poor quality and/or that inhibitory components may be an important factor during diatom blooms, even when significant amounts of alternative prey are available. Our results also suggest that blooms of the silicoflagellate D. speculum may affect copepod reproduction negatively in the sea