A pulse amplitude modulated fluorometer (PAM) was used to investigate photosynthetic activity of m~crophytobenthos on an Intertidal mudflat Spectral ~rradiance measurements Indicate that 7500 of the signal detectable by the PAM onginates In the upper 150 pm of the s e d~m e n t From the photosynthetic electron transport rate (ETR) measurements, it was concluded that the PAM could be used to observe changes in photosynthetic parameters during the day or the season Photoacclimation to lower irrad~ance was ind~cated by changes in the maximum ETR and the saturating photon irradiance parameter I, When cores were exposed to a high photon irradiance for several hours, vertical migration could be followed using reflectance spectra The data also showed that the benthic algae did not seem to expenence photoinhib~tion or COZ limitation To explain this, ~t is hypothes~sed that there is a continuous vertical migration in the top layer of the s e d~m e n t , whele algae can a v o~d photoinhibition due to prolonged periods of high irrad~ance and lack of CO2 by m~grating downwards whlle others migrate upwards KEY WORDS Microphytobenthos Chlorophyll fluorescence Photosynthesis Vert~cal I-rugratlon C-limitation

Microphytobenthlc primary production and biomass were studied in an intertidal flat located in the Westerschelde (SW Netherlands) with a vertical resolution of 1 mm. Short-term variability of primary production and photosynthetic characteristics were compared during low tide in 2 types of sediment: sandy and muddy sands. The changes observed were compared with abiotic factors and the biomass abundance, which was dominated by benthic diatoms. 14C uptake values obtained from incubations In a photosynthetron were used for the construction of P-I (photosynthesis-irradiance) curves. Annual averages indicate that both sediments were equally productive (34.5 * 23.6 mg C m-' h-' and 41.1 2 11.6 mg C m-' h-' for the sands and muddy sands respectively), but production rates were highly variable on monthly time scales and were regulated by different mechanisms. Light and temperature played an important role in determining the production rates, especially in the muddy sediments, where changes in I, (light saturation) were correlated with temperature. I, showed seasonal changes, suggesting that algae adapted to the seasonal light conditions but there was not a significant correlation between the Ik and PAR (photosynthetic available radiation) at any of the stations Vertical migration of the algae, as followed by spectroradiometric measurements, probably accounted for a general absence of photoinhibition. In the sandy sedirnents, production appeared to be limited by the low biomass of algae, due to resuspension and export. The fact that gross oxygen production rates measured on intact cores using microelectrodes were not lower than potential production obtained from 14C fixation suggests that short-term llrnitation of production due to nutrients and/or carbon is not frequent in the microphytobenthos of the Westerschelde.

Several experiments have shown a decrease of growth and calcification of organisms at decreased pH levels but relatively few studies have focused on early life stages which are believed to be more sensitive to environmental disturbances such as hypercapnia. Here, we present experimental data demonstrating that the growth of planktonic mussel (Mytilus edulis) larvae is significantly affected by a decrease of pH to a level expected for the end of the century. Even though there was no significant effect of a 0.25–0.34 pH unit decrease on hatching and mortality rates during the first 2 days of development nor during the following 13-day period prior to settlement, final shells were, respectively, 4.5±1.3 and 6.0±2.3% smaller at pH_NBS~7.8 than at a control pH_NBS of ~8.1. Moreover, a decrease of 12.0±5.4% of shell thickness was observed. More severe impacts were found with a decrease of ~0.5 pH_NBS unit during the first 2 days of development which could be attributed to a decrease of calcification due toslight undersaturation of seawater with respect to aragonite. Indeed, important effects on both hatching and D-veliger shell growth were found. Hatching rates were 24±4% lower while D-veliger shells were 12.7±0.9% smaller at pH_NBS~7.6 than at a control pH_NBS of ~8.1. Although these results show that blue mussel larvae are still able to develop a shell in seawater undersaturated with respect to aragonite, decreases of hatching rates and shell growth suggest a negative impact of ocean acidification on the future survival of bivalve populations potentially leading to significant ecological and economical losses

In this paper we compare measured rates of C-fixation obtained using in situ deployment of a fast repetition rate fluorometer (FRRF) applying two different calculation protocols and a waterPAM fluorometer. The measurements were carried out in the large, shallow Lake IJsselmeer (The Netherlands). Turbidity appears to be the most important factor in explaining photosynthetic parameters. Non-photochemical quenching (NPQ) in the antenna was observed, and the degree of NPQ was positively correlated with the irradiance and mixing conditions. However, NPQ did not affect the functional cross-section. Conversion of photosynthetic electron transport rates (ETR) into rates of carbon fixation requires knowledge of two or three parameters (depending on the protocol and instrument used) to calculate C-fixation from ETR: the electron yield (È e , i.e. the reciprocal of the quantum requirement of PSII), the photosynthetic quotient (PQ) and the number of PSII (n PSII ). Using normally assumed values for these three parameters resulted in overestimation of the measured rate of C-fixation but, in all cases, ETR was linearly related to it. The degree of overestimation was rather constant, despite considerable changes in phytoplankton composition. Spectral correction of fluorescence data caused a reduction of 30% or an increase of 44% in the estimated C-fixation, depending how C-fixation was estimated. We describe a method that allows estimation of the È e /PQ ratio based on a comparison of the quantum efficiencies for C-fixation and PSII and show that, with a single and realistic value for this ratio, primary production could be accurately predicted at different times of the year.

Ocean acidification, due to anthropogenic CO2 absorption by the ocean, may have profound impacts on marine biota. Calcareous organisms are expected to be particularly sensitive due to the decreasing availability of carbonate ions driven by decreasing pH levels. Recently, some studies focused on the early life stages of mollusks that are supposedly more sensitive to environmental disturbances than adult stages. Although these studies have shown decreased growth rates and increased proportions of abnormal development under low pH conditions, they did not allow attribution to pH induced changes in physiology or changes due to a decrease in aragonite saturation state. This study aims to assess the impact of several carbonate-system perturbations on the growth of Pacific oyster (Crassostrea gigas) larvae during the first 3 days of development (until shelled D-veliger larvae). Seawater with five different chemistries was obtained by separately manipulating pH, total alkalinity and aragonite saturation state (calcium addition). Results showed that the developmental success and growth rates were not directly affected by changes in pH or aragonite saturation state but were highly correlated with the availability of carbonate ions. In contrast to previous studies, both developmental success into viable D-shaped larvae and growth rates were not significantly altered as long as carbonate ion concentrations were above aragonite saturation levels, but they strongly decreased below saturation levels. These results suggest that the mechanisms used by these organisms to regulate calcification rates are not efficient enough to compensate for the low availability of carbonate ions under corrosive conditions.