Coastal aquaculture has been growing rapidly in the last decades as a response to the global seafood demand and overexploited wild marine populations. As more sustainable food production practices are sought, the attention on the production of low trophic level organisms has increased significantly. Responsible production of primary producers and primary consumers in coastal areas can help meet the ever-increasing seafood demands and ease burdens on natural resources. However, conditions in coastal areas are not easy to control, and changes in ambient factors can impact coastal aquaculture productivity in various ways. Temperature and pH are factors that are already changing globally and are expected to keep changing in response to climate change.
The growth and chemical composition of primary producers, like algae and seaweed, are influenced by the ambient conditions where they grow. Their cells can chemically adapt to the environment, responding to changes in ambient conditions by producing biomass with different nutritional values. Primary producers are the base of the food chain in aquatic ecosystems, serving as a food source for consumers at higher trophic levels. Herbivores and filter feeders directly depend on the availability of good quality primary producers’ sources, hence, any changes in the nutrient content and growth of primary producers will affect the ability to produce these species. Therefore, to assure sustainable growth of coastal aquaculture, it is important to understand how species of interest are affected by the changes in ambient conditions predicted due to climate change, and how they interact and relate to each other.
Here, the relationship between environmental conditions, primary producers, and primary consumers was explored. The main goal of this study was to understand how changes in temperature and pH can influence the production of herbivore species in coastal areas, by changing the nutritional value of their diets. For this, as a first approximation, a microalgae culture system that maintained multiple pH cultures through automatic addition of CO2 to keep the desired pH was designed, and the effects of temperature and pH on the growth and protein content of two species of marine microalgae were explored. The species Nannochloropsis oculata and Chaetoceros gracilis were selected for this study as they are important species in coastal aquaculture of filter feeder species. The study consisted of growing these two species under two different temperatures, 13°C and 20°C, and two pH levels, 8.2 and 7.6, representing the current and projected ocean conditions, in Northern California, due to climate change, respectively. The highest final cell count, specific growth rate, and protein content were found when both species of algae were grown at 20°C and pH 7.6, indicating the projected conditions caused by climate change did not have negative effects on the marine microalgae tested. The statistical analysis results for all the parameters suggest that temperature has a bigger influence than pH on both species of algae.
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The results of this study suggest that rising ocean temperature and ocean acidification caused by climate change might have positive effects on the protein content and the growth of the marine microalgae studied. Furthermore, temperature seemed to be a more influential factor than pH. In the same way, rising ocean temperature positively affected the protein content of the seaweed studied, however, its growth and condition deteriorated as temperature increased. Therefore, even though dulse growing at 17°C yielded higher growth rates of abalone, keeping this seaweed at higher temperatures will not be sustainable. The different diets used for this study did not affect the nutritional composition of the juvenile red abalone. Finally, higher temperatures due to climate change did not seem to have negative indirect effects on the juvenile red abalone and overall dulse growth rate was the only factor studied that was negatively affected by the predicted conditions due to climate change.
Mata Barboza R., 2023. Effects of climate change on the growth and chemical composition of primary producers and its impacts on coastal aquaculture. PhD thesis, UC Davis, 146 p. Thesis.
