Marja Koski

Development and grazing of Temora longicornis (Copepoda, Calanoida) nauplii during nutrient limited Phaeocystis globosa blooms in mesocosms

The rates of development and food intake of the copepod Temora longicornis (Müller) were studied using artificial blooms of Phaeocystis globosa Scherffel under different conditions of nutrient limitation. Mesocosms with 800 l of natural seawater were manipulated by inoculation with cultured P. globosa and by addition of nitrogen and/or phosphorus, to obtain N- or P-limited blooms of P. globosa. During development and ageing of these blooms, water from the mesocosms was used as medium for incubation of nauplii of T. longicornis. Only moderate rates of naupliar development as well as high rates of mortality were observed, irrespective of major differences of nutrient conditions and density of P. globosa. Grazing by the nauplii on P. globosa seemed to be low, suggesting a low food quality of this alga at all physiological conditions studied. The results of this study indicate a low capability of T. longicornis nauplii for control of nuisance algal blooms caused by P. globosa.

Strain-related physiological and behavioral effects of Skeletonema marinoi on three common planktonic copepods

Three strains of the chain-forming diatom Skeletonema marinoi, differing in their production of polyunsaturated aldehydes (PUA) and nutritional food components, were used in experiments on feeding, egg production, hatching success, pellet production, and behavior of three common planktonic copepods: Acartia tonsa, Pseudocalanus elongatus, and Temora longicornis. The three different diatom strains (9B, 1G, and 7J) induced widely different effects on Acartia tonsa physiology, and the 9B strain induced different effects for the three copepods. In contrast, different strains induced no or small alterations in the distribution, swimming behavior, and turning frequency of the copepods. 22:6(n-3) fatty acid (DHA) and sterol content of the diet typically showed a positive effect on either egg production (A. tonsa) or hatching success (P. elongatus), while other measured compounds (PUA, other long-chain polyunsaturated fatty acids) of the algae had no obvious effects. Our results demonstrate that differences between strains of a given diatom species can generate effects on copepod physiology, which are as large as those induced by different algae species or groups. This emphasizes the need to identify the specific characteristics of local diatoms together with the interacting effects of different mineral, biochemical, and toxic compounds and their potential implications on different copepod species.

Metagenomic insights into zooplankton-associated bacterial communities

Summary Zooplankton and microbes play a key role in the oceans biological cycles by releasing and consuming copious amounts of particulate and dissolved organic matter. Additionally, zooplankton provide a complex microhabitat rich in organic and inorganic nutrients in which bacteria thrive. In this study, we assessed the phylogenetic composition and metabolic potential of microbial communities associated with crustacean zooplankton species collected in the North Atlantic. Using Illumina sequencing of the 16S rRNA gene, we found significant differences between the microbial communities associated with zooplankton and those inhabiting the surrounding seawater. Metagenomic analysis of the zooplankton‐associated microbial community revealed a highly specialized bacterial community able to exploit zooplankton as microhabitat and thus, mediating biogeochemical processes generally underrepresented in the open ocean. The zooplankton‐associated bacterial community is able to colonize the zooplanktons internal and external surfaces using a large set of adhesion mechanisms and to metabolize complex organic compounds released or exuded by the zooplankton such as chitin, taurine and other complex molecules. Moreover, the high number of genes involved in iron and phosphorus metabolisms in the zooplankton‐associated microbiome suggests that this zooplankton‐associated bacterial community mediates specific biogeochemical processes (through the proliferation of specific taxa) that are generally underrepresented in the ambient waters.

Ecological effects of scrubber water discharge on coastal plankton: Potential synergistic effects of contaminants reduce survival and feeding of the copepod Acartia tonsa

To meet the oncoming requirements for lower sulphur emissions, shipping companies can install scrubbers where the exhaust is sprayed with seawater and subsequently discharged to the sea. The discharge water has a pH around 3 and contains elevated concentrations of vanadium, nickel, lead and hydrocarbons. We investigated 1) the threshold concentrations of scrubber discharge water for survival, feeding and reproduction of the copepod Acartia tonsa, 2) whether the effects depend on the exposure route and 3) whether exposure to discharge water can be detected in field-collected organisms. A direct exposure to discharge water increased adult copepod mortality and reduced feeding at metal concentrations which were orders of magnitude lower than the lethal concentrations in previous single-metal studies. In contrast, reproduction was not influenced by dietary uptake of contaminants. Scrubber water constituents could have synergistic effects on plankton productivity and bioaccumulation of metals, although the effects will depend on their dilution in the marine environment.

Copepods Boost the Production but Reduce the Carbon Export Efficiency by Diatoms

The fraction of net primary production that is exported from the euphotic zone as sinking particulate organic carbon (POC) varies notably through time and from region to region. Phytoplankton containing biominerals, such as silicified diatoms have long been associated with high export fluxes. However, recent reviews point out that the magnitude of export is not controlled by diatoms alone, but determined by the whole plankton community structure. The combined effect of phytoplankton community composition and zooplankton abundance on export flux dynamics, were explored using a set of 12 large outdoor mesocosms. All mesocosms received a daily addition of minor amounts of nitrate and phosphate, while only 6 mesocosms received silicic acid (dSi). This resulted in a dominance of diatoms and dinoflagellate in the +Si mesocosms and a dominance of dinoflagellate in the –Si mesocosms. Simultaneously, half of the mesocosms had decreased mesozooplankton populations whereas the other half were supplemented with additional zooplankton. In all mesocosms, POC fluxes were positively correlated to Si/C ratios measured in the surface community and additions of dSi globally increased the export fluxes in all treatments highlighting the role of diatoms in C export. The presence of additional copepods resulted in higher standing stocks of POC, most probably through trophic cascades. However it only resulted in higher export fluxes for the –Si mesocosms. In the +Si with copepod addition (+Si +Cops) export was dominated by large diatoms with higher Si/C ratios in sinking material than in standing stocks. During non-bloom situations, the grazing activity of copepods decrease the export efficiency in diatom dominated systems by changing the structure of the phytoplankton community and/or preventing their aggregation. However, in flagellate-dominated system, the copepods increased phytoplankton growth, aggregation and fecal pellet production, with overall higher net export not always visible in term of export efficiency.

Feeding on dispersed vs. aggregated particles: The effect of zooplankton feeding behavior on vertical flux

Zooplankton feeding activity is hypothesized to attenuate the downward flux of elements in the ocean. We investigated whether the zooplankton community composition could influence the flux attenuation, due to the differences of feeding modes (feeding on dispersed vs. aggregated particles) and of metabolic rates. We fed 5 copepod species—three calanoid, one harpacticoid and one poecilamastoid–microplankton food, in either dispersed or aggregated form and measured rates of respiration, fecal pellet production and egg production. Calanoid copepods were able to feed only on dispersed food; when their food was introduced as aggregates, their pellet production and respiration rates decreased to rates observed for starved individuals. In contrast, harpacticoids and the poecilamastoid copepod Oncaea spp. were able to feed only when the food was in the form of aggregates. The sum of copepod respiration, pellet production and egg production rates was equivalent to a daily minimum carbon demand of ca. 10% body weight-1 for all non-feeding copepods; the carbon demand of calanoids feeding on dispersed food was 2–3 times greater, and the carbon demand of harpacticoids and Oncaea spp. feeding on aggregates was >7 times greater, than the resting rates. The zooplankton species composition combined with the type of available food strongly influences the calculated carbon demand of a copepod community, and thus also the attenuation of vertical carbon flux.