For an increasing number of seaweeds species, land base pond systems are becoming a preferred choice for commercial cultivation due to the high production of biomass they can reach. These systems, commonly named "tumble culture", consist in ponds where seaweeds are in a continuous resuspension in the water column (1m) by air supplied from the bottom of the pond. Nutrients are supplied by periodic fertilization and water renewal. Biomass densities are controlled in order to optimize biomass production. However, maximum production (biomass per unit area per time) is a combination of maximizing biomass per unit area where photosynthesis and growth are not limited. These conditions, however, are dynamic on a daily and seasonal bases. Seaweed growers usually opt for setting fixed seeding biomass densities (usually from 1 to 3 kg m3) and harvesting period (3-4 weeks) where seaweed biomass reaches or is close to the carrying capacity of the system (7-9 kg ww). The purpose of this study was to determine the optimal production of Ulva sp. evaluating its physiological responses and growth during its cultivation in a land-based 40 m3 ponds. Ponds were seeded with 1kg/m3 with a previously selected strain of Ulva sp. Cultures were monitored for four weeks. Pulse fertilization and full water exchange were practiced twice a week. Photosynthesis, respiration, pigments, and nutrient content were measured. Light, temperature, pH, and dissolved inorganic nitrogen (DIN) were simultaneously monitored. Additionally, laboratory experiments were performed to assess the effects of increasing temperature and pH on the seaweed physiology (photosynthesis, nitrogen uptake). DIN (ammonium, nitrate) uptake kinetics were also determined to evaluate Ulva sp capacity to incorporate the fertilizer used during cultivation.
Decrease in photosynthetic capacities due to the low availability of light and inorganic carbon for photosynthesis as biomass increased was observed after the second week. Seaweed specific growth rate was also reduced, resulting in a reduction in biomass productivity. Biomass yield decreased from 300 g/m3/d in the first two weeks to 170 g/m3/d for the 3rd and 4th week of cultivation. Short-term (hours) increments in pH and temperature, which can occur on daily-basis, caused severe inhibition of photosynthesis and nitrogen uptake, as obtained from laboratory experiments. Also, uptake kinetics demonstrated that Ulva sp. incorporates ammonium more efficiently than nitrate. Generally, our results can be directly applied to optimize culture conditions of Ulva sp. Based on these results, biomass production could be optimized by harvesting between the first and second week of cultivation during Spring and early Summer providing a 20% net increase in biomass production in a four-week period. A similar approach can be applied to know the best harvesting frequencies in the Fall and Winter.