Seasonal dynamics of the carbonate system in the Western English Channel

We present over 900 carbonate system observations collected over four years (2007–2010) in the Western English Channel (WEC). We determined CO₂ partial pressure (pCO₂), Total Alkalinity (TA) and Dissolved Inorganic Carbon (DIC) along a series of 40 km transects, including two oceanographic stations (L4 and E1) within a sustained coastal observatory. Our data follow a seasonal pattern of CO₂ undersaturation from January to August, followed by supersaturation in September-October and a return to near-equilibrium thereafter. This pattern is explained by the interplay of thermal and biological sinks in winter and spring-summer respectively, followed by the breakdown of stratification and mixing with deeper, high-CO₂ water in autumn. The drawdown of DIC and inorganic N between March and June with a C:N ratio of 8.7–9.5 was consistent with carbon over-consumption during phytoplankton growth. Monthly mean surface pCO₂ was strongly correlated with depth integrated chlorophyll a highlighting the importance of subsurface chlorophyll a maxima in controlling C-fluxes in shelf seas. Mixing of seawater with riverine freshwater in near-shore samples caused a reduction in TA and the saturation state of calcite minerals, particularly in winter. Our data show that the L4 and E1 oceanographic stations were small, net sinks for atmospheric CO₂ over an annual cycle (−0.52±0.66 mol C m⁻² y⁻¹ and −0.62±0.49 mol C m⁻² y⁻¹ respectively).

Response of Nodularia spumigena to pCO2 – Part 2: Exudation and extracellular enzyme activities

The filamentous and diazotrophic cyanobacterium Nodularia spumigena plays a major role in the productivity of the Baltic Sea as it forms extensive blooms regularly. Under phosphorus limiting conditions Nodularia spumigena has a high enzyme affinity for dissolved organic phosphorus (DOP) by production and release of alkaline phosphatase. Additionally, it is able to degrade proteinaceous compounds by expressing the extracellular enzyme leucine aminopeptidase. As atmospheric CO₂ concentrations are increasing, we expect marine phytoplankton to experience changes in several environmental parameters including pH, temperature, and nutrient availability. The aim of this study was to investigate the combined effect of CO₂-induced changes in seawater carbonate chemistry and of phosphate deficiency on the exudation of organic matter, and its subsequent recycling by extracellular enzymes in a Nodularia spumigena culture. Batch cultures of Nodularia spumigena were grown for 15 days aerated with three different pCO₂ levels corresponding to values from glacial periods to future values projected for the year 2100. Extracellular enzyme activities as well as changes in organic and inorganic compound concentrations were monitored. CO₂ treatment–related effects were identified for cyanobacterial growth, which in turn was influencing exudation and recycling of organic matter by extracellular enzymes. Biomass production was increased by 56.5% and 90.7% in the medium and high pCO₂ treatment, respectively, compared to the low pCO₂ treatment and simultaneously increasing exudation. During the growth phase significantly more mucinous substances accumulated in the high pCO₂ treatment reaching 363 μg Gum Xanthan eq l⁻¹ compared to 269 μg Gum Xanthan eq l⁻¹ in the low pCO₂ treatment. However, cell-specific rates did not change. After phosphate depletion, the acquisition of P from DOP by alkaline phosphatase was significantly enhanced. Alkaline phosphatase activities were increased by factor 1.64 and 2.25, respectively, in the medium and high compared to the low pCO₂ treatment. In conclusion, our results suggest that Nodularia spumigena can grow faster under elevated pCO₂ by enhancing the recycling of organic matter to acquire nutrients.