Posts Tagged 'primary production'

Dynamics of benthic metabolism, O2, and pCO2 in a temperate seagrass meadow

Seagrass meadows play an important role in “blue carbon” sequestration and storage, but their dynamic metabolism is not fully understood. In a dense Zostera marina meadow, we measured benthic O2 fluxes by aquatic eddy covariance, water column concentrations of O2, and partial pressures of CO2 (pCO2) over 21 full days during peak growing season in April and June. Seagrass metabolism, derived from the O2 flux, varied markedly between the 2 months as biomass accumulated and water temperature increased from 16°C to 28°C, triggering a twofold increase in respiration and a trophic shift of the seagrass meadow from being a carbon sink to a carbon source. Seagrass metabolism was the major driver of diurnal fluctuations in water column O2 concentration and pCO2, ranging from 173 to 377 μmol L−1 and 193 to 859 ppmv, respectively. This 4.5‐fold variation in pCO2 was observed despite buffering by the carbonate system. Hysteresis in diurnal water column pCO2 vs. O2 concentration was attributed to storage of O2 and CO2 in seagrass tissue, air–water exchange of O2 and CO2, and CO2 storage in surface sediment. There was a ~ 1:1 mol‐to‐mol stoichiometric relationship between diurnal fluctuations in concentrations of O2 and dissolved inorganic carbon. Our measurements showed no stimulation of photosynthesis at high CO2 and low O2 concentrations, even though CO2 reached levels used in IPCC ocean acidification scenarios. This field study does not support the notion that seagrass meadows may be “winners” in future oceans with elevated CO2 concentrations and more frequent temperature extremes.

Continue reading ‘Dynamics of benthic metabolism, O2, and pCO2 in a temperate seagrass meadow’

Kelp beds and their local effects on seawater chemistry, productivity, and microbial communities

Kelp forests are known as key habitats for species diversity and macroalgal productivity; however, we know little about how these biogenic habitats interact with seawater chemistry and phototroph productivity in the water column. We examined kelp forest functions at three locales along the Olympic Peninsula of Washington state by quantifying carbonate chemistry, nutrient concentrations, phytoplankton productivity, and seawater microbial communities inside and outside of kelp beds dominated by the canopy kelp species Nereocystis luetkeana and Macrocystis pyrifera. Kelp beds locally increased the pH, oxygen, and aragonite saturation state of the seawater, but lowered seawater inorganic carbon content and total alkalinity. While kelp beds depleted nitrate and phosphorus concentrations, ammonium and DOC concentrations were enhanced. Kelp beds also decreased chlorophyll concentrations and carbon fixed by phytoplankton, although kelp carbon fixation more than compensated for any difference in phytoplankton production. Kelp beds also entrained distinct microbial communities, with higher taxonomic and phylogenetic diversity compared to seawater outside of the kelp bed. Kelp forests thus had significant effects on seawater chemistry, productivity and the microbial assemblages in their proximity. Thereby, the diversity of pathways for carbon and nitrogen cycling was also enhanced. Overall, these observations suggest that the contribution of kelp forests to nearshore carbon and nitrogen cycling is greater than previously documented.

Continue reading ‘Kelp beds and their local effects on seawater chemistry, productivity, and microbial communities’

In situ response of Antarctic under-ice primary producers to experimentally altered pH

Elevated atmospheric CO2 concentrations are contributing to ocean acidification (reduced seawater pH and carbonate concentrations), with potentially major ramifications for marine ecosystems and their functioning. Using a novel in situ experiment we examined impacts of reduced seawater pH on Antarctic sea ice-associated microalgal communities, key primary producers and contributors to food webs. pH levels projected for the following decades-to-end of century (7.86, 7.75, 7.61), and ambient levels (7.99), were maintained for 15 d in under-ice incubation chambers. Light, temperature and dissolved oxygen within the chambers were logged to track diurnal variation, with pH, O2, salinity and nutrients assessed daily. Uptake of CO2 occurred in all treatments, with pH levels significantly elevated in the two extreme treatments. At the lowest pH, despite the utilisation of CO2 by the productive microalgae, pH did not return to ambient levels and carbonate saturation states remained low; a potential concern for organisms utilising this under-ice habitat. However, microalgal community biomass and composition were not significantly affected and only modest productivity increases were noted, suggesting subtle or slightly positive effects on under-ice algae. This in situ information enables assessment of the influence of future ocean acidification on under-ice community characteristics in a key coastal Antarctic habitat.

Continue reading ‘In situ response of Antarctic under-ice primary producers to experimentally altered pH’

Ocean acidification and high irradiance stimulate growth of the Antarctic cryptophyte Geminigera cryophila

Ecophysiological studies on Antarctic cryptophytes to assess whether climatic changes such as ocean acidification and enhanced stratification affect their growth in Antarctic coastal waters in the future are lacking so far. This is the first study that investigated the combined effects of increasing availability of pCO2 (400 and 1000 µatm) and irradiance (20, 200 and 500 μmol photons m−2 s−1) on growth, elemental composition and photophysiology of the Antarctic cryptophyte Geminigera cryophila. Under ambient pCO2, this species was characterized by a pronounced sensitivity to increasing irradiance with complete growth inhibition at the highest light intensity. Interestingly, when grown under high pCO2 this negative light effect vanished and it reached highest rates of growth and particulate organic carbon production at the highest irradiance compared to the other tested experimental conditions. Our results for G. cryophila reveal beneficial effects of ocean acidification in conjunction with enhanced irradiance on growth and photosynthesis. Hence, cryptophytes such as G. cryophila may be potential winners of climate change, potentially thriving better in more stratified and acidic coastal waters and contributing in higher abundance to future phytoplankton assemblages of coastal Antarctic waters.

Continue reading ‘Ocean acidification and high irradiance stimulate growth of the Antarctic cryptophyte Geminigera cryophila’

Grazers increase the sensitivity of coralline algae to ocean acidification and warming

Highlights

  • Stimulation of the primary production and calcification of corallines by grazing
  • Different response of maerl between winter and summer conditions
  • High vulnerability of corallines to ocean acidification in the presence of grazers

Abstract

Coralline algae are expected to be adversely impacted by ocean acidification and warming. Most research on these algae has involved experiments on isolated species, without considering species interactions, such as grazing. This myopic view is challenging because the impact of climate change on coralline algae will depend on the direct impacts on individual coralline species and the indirect effects of altered interactions with other species. Here, we tested the influence of grazing on the response of the coralline alga Lithothamnion corallioides to near-future ocean acidification and warming. Two three-month experiments were performed in the winter and summer seasons in mesocosms under crossed conditions of pCO2 (ambient and high pCO2) and temperature (ambient and +3 °C) in the presence and absence of grazers. In the winter, L. corallioides photosynthesis decreased with rising temperature in the presence of grazers, while calcification increased. It is likely that increased calcification may act as a structural protection to prevent damage from grazing. However, increasing calcification rates in the presence of grazers may be detrimental to other physiological processes, such as photosynthesis. In the summer, L. corallioides primary production, respiration, and calcification were higher in the presence of grazers than in their absence. Light calcification rates were reduced under high pCO2 in the presence of grazers only. Moreover, dark calcification rates were more adversely affected by pCO2 increase in the presence of grazers. Through their feeding activity, grazers may alter the structural integrity of thalli and increase the sensitivity of coralline algae to ocean acidification. Our results indicate that both season and grazing play a key role in the response of L. corallioides to acidification and warming. Seasonal variations and species interactions are thus critical to consider to make ecologically relevant predictions of the effects of future environmental changes.

Continue reading ‘Grazers increase the sensitivity of coralline algae to ocean acidification and warming’

Varying conditions in intertidal pools: high resolution pH dynamics and primary production

Most studies designed to assess the effects of ocean acidification take place in coastal and intertidal environments, which are characterized by a great variability of its physical and chemical parameters. However, a great number of these studies use fixed pH levels predicted for the future, disregarding natural pH oscillations. In this work we studied the pH oscillations and primary productivity of intertidal rockpools in two rocky shore areas. To provide high resolution continuous pH data we used an autonomous pH measuring system which consisted of a pH sensor, a data logger and a battery encased in a waterproof container. Oxygen concentration and primary production from phytoplankton and macro- phytobentos were also measured. We found a range of pH variation in the pools of 0.07 pH units/day when water dynamics was high and of 0.26 pH units/day when conditions were more stable. Carbonate systems parameters, temperature and oxygen concentration were related and they responded to the day / night cycle and hydrodynamic conditions. We suggest that these natural oscillations in pH and temperature must be taken into account in ocean acidifications studies in order to obtain more accurate results.

Continue reading ‘Varying conditions in intertidal pools: high resolution pH dynamics and primary production’

Insensitivities of a subtropical productive coastal plankton community and trophic transfer to ocean acidification: results from a microcosm study

Highlights
• Lower apparent growth was observed under elevated CO2 of 1000 μatm.

• Primary production and trophic transfer were unaffected by high CO2.

• Fatty acid profiles of phyto-/zooplankton were unaffected by ocean acidification.

Abstract
Ocean acidification (OA) has potential to affect marine phytoplankton in ways that are partly understood, but there is less knowledge about how it may alter the coupling to secondary producers. We investigated the effects of OA on phytoplankton primary production, and its trophic transfer to zooplankton in a subtropical eutrophic water (Wuyuan Bay, China) under present day (400 μatm) and projected end-of-century (1000 μatm) pCO2 levels. Net primary production was unaffected, although OA did lead to small decreases in growth rates. OA had no measurable effect on micro-/mesozooplankton grazing rates. Elevated pCO2 had no effect on phytoplankton fatty acid (FA) concentrations during exponential phase, but saturated FAs increased relative to the control during declining phase. FA profiles of mesozooplankton were unaffected. Our findings show that short-term exposure of plankton communities in eutrophic subtropical waters to projected end-of-century OA conditions has little effect on primary productivity and trophic linkage to mesozooplankton.

Continue reading ‘Insensitivities of a subtropical productive coastal plankton community and trophic transfer to ocean acidification: results from a microcosm study’


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