Effects of season, depth and pre-cultivation fertilizing on Ulva growth dynamics offshore the Eastern Mediterranean Sea

Abstract

Offshore macroalgae production could provide an alternative source of biomass for food, materials and energy. However, the offshore environment in general, specifically the Eastern Mediterranean Sea (EMS) offshore, is a high energy and low nutrients environment, thus challenging for macroalgae farming. In this study, we experimentally investigated the impact of season, depth, and pre-cultivation fertilization duration on the growth rates and chemical composition of offshore Ulva biomass, and developed a predictive model tailored to offshore conditions, capable of estimating both biomass growth rate and nitrogen content. Specifically, we measured Ulva biomass growth rate and internal nitrogen in the nitrogen-poor EMS a few kilometers offshore the Israeli coast at various depths and on-shore pre-cultivation fertilization schedules. Based on these data, we constructed a predictive cultivation model of Ulva offshore growth, which allows for the optimization of fertilization requirements for offshore cultivation. This study provides new insights on the effects of seasonality, depth, and pre-cultivation fertilization duration on growth rates and chemical composition of offshore Ulva sp. biomass production.

Figure 1

Top row: experimental flow diagram. (a) Image of the onshore continuous fertilizing of the Ulva sp. stock in an MPBR system in the aquaculture center in Michmoret, (b) images of: the offshore experiments site on the North-Western marking buoy of the Lev-Yam fish cages and the installation of the cultivation rings in a depth of 1m and in a depth of 5m, (c) onshore rapid fertilizing in aerated tanks, used between offshore cultivation periods. Bottom row: illustration images: (d) map of cultivation site offshore Michmoret, (e) illustration of a single cultivation ring and attached cages, and (f) three cages stocked with Ulva biomass installed on the cultivation system.

Figure 2

Experimental results of daily growth rate (a,c,e) and internal N (b,d,f) of Ulva sp. macroalgae cultivated in cages offshore the EMS under different conditions of depth (top row), pre-cultivation fertilizing (middle row) and experiment date (bottom row). Representative images of cages with Ulva sp. biomass after cultivation in depths of 1 m (top) and 5 m (bottom) are presented in the right column (g). Top row: 1 m depth (light blue) vs 5 m depth (dark blue). Analysis included only Ulva sp. cultivated after continuous nutrient enrichment in experiments #1 and #2. Sample sizes: 15 for DGR and 4 for internal N for each depth. Asterisks indicate statistical significance of difference with ****p < 0.0001, calculated by the two-tailed Mann–Whitney U test. Middle row: rapid (1-day) pre-cultivation nutrient enrichment (light blue) vs continuous pre-cultivation nutrient enrichment (dark blue). Sample sizes: rapid nutrient enrichment: 22 for DGR and 7 for internal N. continuous nutrient enrichment: 54 for DGR and 21 for internal N. Asterisks indicate statistical significance of difference with ****p < 0.0001, calculated by the two-tailed Mann–Whitney U test. Bottom row: DGR (e) and internal N (f) at different experiment dates. On the left, Group A is statistically different from group B, in a significance of p < 0.05, calculated by the post-hoc Dunn’s test with the Bonferroni adjustment method for pairwise comparison. On the right, light blue dots present internal N at the beginning of each experiment and dark blue dots represent internal N at the end of each experiment. Analysis included only Ulva sp. cultivated after continuous nutrient enrichment in a depth of 5 m. From the preliminary run (May 19) we show only internal N results, as the DGR measurements are meaningless due to biomass losses. Sample sizes: 6–12 for DGR and 2–8 for internal N for the different experiments.

Figure 3

Modeled (marked by x) vs measured (in boxes) biomass (a) and internal N (b) of Ulva sp. cultivated for a period of 7–10 days in cages that were installed in a depth of 5 m cages offshore the EMS after continuous nutrient enrichment in experiments #1, #2 and #4. In experiment #4, the red x presents model predictions using a constant $N_{\mathit{ext}}$ of 0.75 µM N and the black x presents model predictions using a changing $N_{\mathit{ext}}$, as described in Table 5. (c) Illustrated sensitivity of simulated biomass production (black circles) and N content (blue stars) to model parameters, as measured by the Sobol method, in the offshore cultivation system.