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Question 3 — Dissolved oxygen dynamics

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Question 3 — Dissolved oxygen dynamics.

A marine biologist colleague of yours has measured temperature and dissolved oxygen (O,) concentrations in a shallow bay. He has given you the data as mean daily values of Temp (°C) and dissolved oxygen (DO in mg/kg) -— see attached Table. He is interested in the dynamics of oxygen in this system and is asking your opinion on how to interpret the dissolved O₂ time series. He did not measure salinity but thinks it should be about 30.

a. Determine the expected DO concentrations if the surface waters were at equilibrium with the atmosphere for each day assuming given salinity, measured temperature and 1 atmosphere of air pressure (assume air is saturated with water vapor). These are DO_satuation concentrations (hint: there are different ways to arrive at this answer, so make sure to clearly explain the source of your data -see comment at end of exam)

b. Plot the concentrations of DO_measured ANA DO_saturation as a function of days in September. Explain how these two variables change in relationship to the water temperature trends.

c. Calculate the difference from saturation as a percent (ΔDO (%) = [(DO_meas-DO_sat)/DO_sat] x100). Plot ΔDO as a function of days in September. What processes explain the observed concentrations of DO_measured and ΔDO?

d. After giving your report to your colleague; he mentions that salinity was unlikely to have remained constant after all. He tells you that while salinity was ~30 on the first 10 days of the study, it slowly increased as the month progressed, so that by day 21 salinity in the bay was 33 and by the end of the month (day 29) it was up to full ocean value of 35. Use this information to revise your estimates of DO_saturation and %ΔDO for the those days. Do your explanations regarding the processes controlling DO in this system change, if so how?

Question 4

You are investigating a spring algae bloom that formed in the North Atlantic, when the seasonal mixed layer shoaled to about 50 m. This bloom lasted for one month and was dominated by coccolithophores that were rapidly exported. Fluxes of sinking organic carbon during these blooms was 50 mmol C m^-2 d^-1. The organic carbon: calcium carbonate ratio for this alga is about 4:1.

a) Given that the initial dissolved inorganic carbon (DIC) and carbonate alkalinity (Ac) are 2,100 µmol L^-1 and 2,300 µeq L^-1 respectively, estimate [CO₃²⁺] and pH? (pK1’ = 5.85; pK2’ = 8.97). Carefully state your assumptions.

b) Assume the rate of CO, gas exchange with the atmosphere is slow compared to export of carbon as sinking particles, and make time course plots of DIC and Ac over the course of the bloom.

c) Approximate the total change in [CO₃²⁺] during the bloom, stating any assumptions made in the calculation. Did pH rise or fall? From a physiological perspective, do the changing conditions of the bloom favor or disfavor calcium carbonate precipitation?