Tara M. Clemente, Daniela Böttjer, Sam T. Wilson, Karin M. Björkman and David M. Karl Center for Microbial Oceanography: Research and Education, University of Hawai’i at Mānoa

Abstract

Theory & Prediction Experimental Results

The development of seawater air conditioning (SWAC) and ocean thermal energy conversion (OTEC) industries in coastal marine habitats may have several unintended environmental consequences, including but not limited to the introduction of deep-sea nutrients (e.g. nitrate, phosphate, silicate), dissolved gases (e.g. CO2, N2O), and microbial genomes into the surface ocean. We have recently conducted several experiments on multiple scales (20-60,000L) to investigate potential impacts of enhanced ocean upwelling on microbial processes. Experimental mixtures of deep and surface seawater collected from Hawaiian waters always leads to a phytoplankton bloom although the rate and intensity of bloom formation are not always the same. We believe that seasonal variations in light, nutrient input and

In December 2011, we conducted the first ever 60,000 liter open ocean mesocosm experiment to test the prediction that if phosphate was removed from the effluent prior to discharge, it would prevent phytoplankton bloom formation. We initially selected phosphate rather than nitrate, in part because of the potential in Hawaiian waters for the growth of N2-fixing cyanobacteria that might otherwise be selected for in a “phosphate” only treatment.

Chl a and primary production measurements increased with the addition of phosphate, However the removal of phosphate suppressed the formation of a large phytoplankton bloom.

Conclusion & Future Prospectus

Light plus nutrients (nitrate and phosphate) leads to bloom (high chlorophyll) as detected by ocean color sensors on satellites.

Depth profiles (surface to 1,000m) of key environmental parameters in Hawaiian waters as determined by the Hawaii Ocean Time-series program. Mass, nutrient, gas and microbe displacement are key things to consider during enhanced upwelling operations.

Variable Surface Ocean

(0-50m)

Deep Ocean

(500-1000m) Temperature High Low Nitrate Very Low High Phosphate Low High Inorganic carbon (CO2) High Very High Oxygen Very High Low Nitrous oxide (N2O) Low High Archaea Low High Deep-sea genes Very Low Very High

Poster #: 1555

Traditionally, incubation experiments to study planktonic processes have been conducted in bottles ranging from 1-20L. However, there are certain recognized drawbacks of incubations on such a small scale such as the ‘bottle effect’ or the exclusion of ‘rare organisms’. Open ocean mesocosms allow scaling from traditional bottle incubations by enclosing a much larger volume of water, thus minimizing these drawbacks.

To monitor and compare the microbial response of nutrient loading via deep water intrusion, seawater was collected from a target outflow located off Waikiki in the proposed SWAC location. To mimic nutrient loading along a spatial gradient dilutions of 15%, 10%, 5% and 1% of deep sea water (DSW) was added to 85% whole surface seawater (30m) and incubated at 30m light levels.

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N+N A (Control) B (15% DSW) C (10% DSW) D (5% DSW) E (1% DSW)

Seawater Air Conditioning

Scaling

With SWAC cold deep seawater, is piped from the ocean then pumped through a heat exchanger to cool a closed loop of fresh water. The chilled water is then piped throughout an entire building district, cooling the buildings that rely on it for their air conditioning. The warmed seawater is returned to the ocean through another pipe and diffuser located at a shallower depth.

5% DSW 15% DSW

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15% DSW

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5% 0.2μm SSW 10% 0.2μm SSW 10% DSW 1% DSW

14% 0.2μm SSW

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surface seawater only

nitrate, phosphate, silicate, trace metals and vitamins

nitrate, silicate, trace metals and vitamins

Experiment Duration: 6days

Nutrient (N+N, SRP and SiO2) concentrations were highest in the 15% DSW addition. N+N and SRP were rapidly drawn down by the 48hr time point in all additions except in the 15% DSW addition which occurred at the 72hr time point.

Chlorophyll a concentrations peaked in all DSW additions at 48hrs and then began to decrease, except in the 15% DSW addition which peaked at 72hrs. The control and 1% DSW addition were dominated by phytoplankton in the 0.2μm size fraction. The 15%, 10% and 5% DSW addition were dominated by 0.2μm phytoplankton until the 48hr time point, then shifted to being dominated by 10μm phytoplankton for the remaining time points. Particulate Silicate increased in all treatments after 48hrs, indicating a potential increase in diatom abundance and were highest in the 15% DSW addition.

Prediction of dynamics of chlorophyll vs. time after nutrient addition showing the selection for large phytoplankton (diatoms) over the smaller forms (Prochlorococcus).

the structure of the phytoplankton community play a large role in determining bloom dynamics. Small scale (20L) experiments examining the microbial response to nutrient loading off Waikiki showed an increase in all phytoplankton (Chl a) size classes with the largest contribution from the >10μm size class. Particulate silicate increase accounted for 90% of the measured decrease in dissolved silicate and nearly all phosphate was removed. These findings along with results from a large scale experiment conducted in ocean mesocosms (60,000L) indicate that the removal of phosphate prior to the effluent discharge may be an effective means to minimize environmental impacts, especially nutrient-induced phytoplankton blooms.

Both OTEC and SWAC will use nutrient-rich seawater as a working fluid and if eventually discharged in to the sunlit waters (0-175m) ecological theory would predict the formation of a phytoplankton bloom, most likely of large phytoplankton including diatoms (from Karl 2007, Nature Reviews Microbiology). Variations in light, nutrient and the structure

of the phytoplankton community play a large role in determining bloom dynamics. Nutrient loading showed an increase in all phytoplankton (Chl a) with the largest contribution from the >10μm size class. The removal of phosphate suppressed the formation of a large phytoplankton bloom and therefore may be a treatment option for effluents from OTEC and SWAC before discharge to the environment.