494Engel et al. AbstractWe studied the direct effects of CO 2 and related changes in seawater carbonate chemistry on marine planktonic organisms in a mesocosm experiment. In nine outdoor enclosures (ϳ11 m 3 each), the partial pressure of CO 2 (pCO 2 ) in the seawater was modified by an aeration system. The triplicate mesocosm treatments represented low (ϳ190 parts per million by volume (ppmV) CO 2 ), present (ϳ410 ppmV CO 2 ), and high (ϳ710 ppmV CO 2 ) pCO 2 conditions. After initial fertilization with nitrate and phosphate a bloom dominated by the coccolithophorid Emiliania huxleyi occurred simultaneously in all of the nine mesocosms; it was monitored over a 19-day period. The three CO 2 treatments assimilated nitrate and phosphate similarly. The concentration of particulate constituents was highly variable among the replicate mesocosms, disguising direct CO 2 -related effects. Normalization of production rates within each treatment, however, indicated that the net specific growth rate of E. huxleyi, the rate of calcification per cell, and the elemental stoichiometry of uptake and production processes were sensitive to changes in pCO 2 . This broad influence of CO 2 on the E. huxleyi bloom suggests that changes in CO 2 concentration directly affect cell physiology with likely effects on the marine biogeochemistry.

The aim of t h~s study was to quantify zooplankton feeding interactions and copepod reproduction dunng blooms of dlatoms and flagellates (lncludlng E m~l l a n~a huxleyl) In fertilised mesocosms A number of mlcrozooplankton grazlng (dilution senes), copepod f e e d~n g (bottle ~ncubation) and egg productlon experiments were performed during a 4 \vk summer penod Mlcrozooplankton (malnly ciliates) peaked dunng an initial bloom d o m~n a t e d by the diatom Skeletonema costatum and flagellates 210 pm w h~c h apparently became grazer-controlled Mauimum grazing rates were 1 5 to 1 8 d.' for diatoms, the calcifying haptophyte Emlllan~a huxleyl and flagellates 2 to 10 pm, and 65 to 80% of the average standing stock of these algae were removed dally Dunng a subsequent bloom of E h u x l e y~ the m~crozooplankton composition changed and ~t s biomass decreased to <25 % and the daily turnover of dlatoms and E huxleyl fell to ca 50% In contrast to other algae E huxleyl spec~fic growth was never surpassed by microzooplankton grazlng The copepod C finn~archlcus (CV and CV1 females) preferred cihates 230 pm, but ciliates <30 pm, dlatoms and rotlfers were also occas~onally preyed upon at h~g h rates E h u x l e y~ was barely Ingested at low concentrations (0 4 to 6 X 105 cells I-') but was cleared at 106 m1 lnd -' d ' at peak concentrations (1 2 X 107 cells I-') It then made up 74 % of total carbon Ingestion Although copepod lngest~on rates were slmilar dunng blooms of diatoms and E huxleyl, egg productlon rates were s~gnlficantly higher dunng blooms of the latter, and mesozooplankton biomass increased 3 tlmes more In mesocosms dominated by E huxleyl compared to mesocosms w~t h dlatom blooms at slmllar algal blomass Impact by copepods on the phytoplankton development was m a n l y induced lndlrectly by selective predation on the m~crozooplankton A method to correct copepod feeding rate measurements for errors d u e to loss of n~~crozooplankton grazing In the ~n c u b a t~o n bottles IS presented

SummaryChytridiomycota, often referred to as chytrids, can be virulent parasites with the potential to inflict mass mortalities on hosts, causing e.g. changes in phytoplankton size distributions and succession, and the delay or suppression of bloom events. Molecular environmental surveys have revealed an unexpectedly large diversity of chytrids across a wide range of aquatic ecosystems worldwide. As a result, scientific interest towards fungal parasites of phytoplankton has been gaining momentum in the past few years. Yet, we still know little about the ecology of chytrids, their life cycles, phylogeny, host specificity and range. Information on the contribution of chytrids to trophic interactions, as well as coevolutionary feedbacks of fungal parasitism on host populations is also limited. This paper synthesizes ideas stressing the multifaceted biological relevance of phytoplankton chytridiomycosis, resulting from discussions among an international team of chytrid researchers. It presents our view on the most pressing research needs for promoting the integration of chytrid fungi into aquatic ecology.

Global pressures on freshwater ecosystems are high and rising. Viewed primarily as a resource for humans, current practices of water use have led to catastrophic declines in freshwater species and the degradation of freshwater ecosystems, including their genetic and functional diversity. Approximately three‐quarters of the world's inland wetlands have been lost, one‐third of the 28 000 freshwater species assessed for the International Union for Conservation of Nature (IUCN) Red List are threatened with extinction, and freshwater vertebrate populations are undergoing declines that are more rapid than those of terrestrial and marine species. This global loss continues unchecked, despite the importance of freshwater ecosystems as a source of clean water, food, livelihoods, recreation, and inspiration. The causes of these declines include hydrological alterations, habitat degradation and loss, overexploitation, invasive species, pollution, and the multiple impacts of climate change. Although there are policy initiatives that aim to protect freshwater life, these are rarely implemented with sufficient conviction and enforcement. Policies that focus on the development and management of fresh waters as a resource for people almost universally neglect the biodiversity that they contain. Here we introduce the Alliance for Freshwater Life, a global initiative, uniting specialists in research, data synthesis, conservation, education and outreach, and policymaking. This expert network aims to provide the critical mass required for the effective representation of freshwater biodiversity at policy meetings, to develop solutions balancing the needs of development and conservation, and to better convey the important role freshwater ecosystems play in human well‐being. Through this united effort we hope to reverse this tide of loss and decline in freshwater biodiversity. We introduce several short‐ and medium‐term actions as examples for making positive change, and invite individuals, organizations, authorities, and governments to join the Alliance for Freshwater Life.

Predicting the ocean's role in the global carbon cycle requires an understanding of the stoichiometric coupling between carbon and growth-limiting elements in biogeochemical processes. A recent addition to such knowledge is that the carbon/nitrogen ratio of inorganic consumption and release of dissolved organic matter may increase in a high-CO(2) world. This will, however, yield a negative feedback on atmospheric CO(2) only if the extra organic material escapes mineralization within the photic zone. Here we show, in the context of an Arctic pelagic ecosystem, how the fate and effects of added degradable organic carbon depend critically on the state of the microbial food web. When bacterial growth rate was limited by mineral nutrients, extra organic carbon accumulated in the system. When bacteria were limited by organic carbon, however, addition of labile dissolved organic carbon reduced phytoplankton biomass and activity and also the rate at which total organic carbon accumulated, explained as the result of stimulated bacterial competition for mineral nutrients. This counterintuitive 'more organic carbon gives less organic carbon' effect was particularly pronounced in diatom-dominated systems where the carbon/mineral nutrient ratio in phytoplankton production was high. Our results highlight how descriptions of present and future states of the oceanic carbon cycle require detailed understanding of the stoichiometric coupling between carbon and growth-limiting mineral nutrients in both autotrophic and heterotrophic processes.

Abstract. Increasing atmospheric carbon dioxide (CO 2 ) concentrations due to anthropogenic fossil fuel combustion are currently changing the ocean's chemistry. Increasing oceanic [CO 2 ] and consequently decreasing seawater pH have the potential to significantly impact marine life. Here we describe and analyze the build-up and decline of a natural phytoplankton bloom initiated during the 2005 mesocosm Pelagic Ecosystem CO 2 Enrichment study (PeECE III). The draw-down of inorganic nutrients in the upper surface layer of the mesocosms was reflected by a concomitant increase of organic matter until day t 11 , the peak of the bloom. From then on, biomass standing stocks steadily decreased as more and more particulate organic matter was lost into the deeper layer of the mesocosms. We show that organic carbon export to the deeper layer was significantly enhanced at elevated CO 2 . This phenomenon might have impacted organic matter remineralization leading to decreased oxygen concentrations in the deeper layer of the high CO 2 mesocosms as indicated by deep water ammonium concentrations. This would have important implications for our understanding of pelagic ecosystem functioning and future carbon cycling.

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.KEY WORDS: Calanus finmarchicus · Calanus helgolandicus · Microzooplankton · Grazing methods · Clearance · Ingestion · Faecal pellet production · Natural plankton · Bottle effect Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 221: [59][60][61][62][63][64][65][66][67][68][69][70][71][72][73][74][75] 2001 Peterson & Dam 1996, Verity & Paffenhöfer 1996, Nejstgaard et al. 1997, and this study). Quantification of copepod feeding rates in the field by methods based on gut pigment (Mackas & Bohrer 1976), 14 C-labeled algae, or plant pigment analysis by HPLC (e.g. Kleppel & Pieper 1984, MeyerHarms et al. 1999) is limited to herbivory, and may substantially underestimate total zooplankton ingestion rates and bias prey selectivity estimates. Such data must be treated with caution. The potential problem with food web interactions in incubation experiments as discussed here may further limit the value of phytoplankton-based methods, if not corrected.The dual labelling technique (Roman & Rublee 1981, Roman & Gauzens 1997) yields data on zooplankton omnivory in situ. However, this method does not give detailed data on feeding selectivity and has a number of potential problems, including the fact that grazing on algae ca...

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