The Importance of Phytoplankton Biomolecule Availability for Secondary Production

Abstract

The growth and reproduction of animals is affected by their access to resources. In aquatic ecosystems, the availability of essential biomolecules for filter-feeding zooplankton depends greatly on phytoplankton. Here, we analyzed the biochemical composition, i.e., the fatty acid, sterol and amino acid profiles and concentrations as well as protein, carbon, nitrogen, and phosphorus content of 17 phytoplankton monocultures representing the seven most abundant phytoplankton classes in boreal and sub-arctic lakes. To examine how the differences in the biochemical composition between phytoplankton classes affect their nutritional quality for consumers, we assessed the performance of Daphnia, on these diets. Furthermore, we defined the most important biomolecules regulating the somatic growth and reproduction of Daphnia, expecting that higher concentrations of certain biomolecules are needed for reproduction than for growth. Finally, we combined these results with phytoplankton field data from over 900 boreal and sub-arctic lakes in order to estimate whether the somatic growth of Daphnia is sterol-limited when the natural phytoplankton communities are cyanobacteria-dominated. Our analysis shows that Daphnia grows best with phytoplankton rich in sterols, ω-3 fatty acids, protein, and amino acids. Their reproduction follows food sterol and ω-3 concentration as well as C:P-ratio being two times higher in Daphnia feeding on cryptophytes than any other diet. Interestingly, we found that a high dietary ω-6 fatty acid concentration decreases both somatic growth and reproduction of Daphnia. When combined with phytoplankton community composition field data, our results indicate that zooplankton is constantly limited by sterols in lakes dominated by cyanobacteria (≥40% of total phytoplankton biomass), and that the absence of cryptophytes can severely hinder zooplankton production in nature.

Terrestrial organic matter quantity or decomposition state does not compensate for its poor nutritional quality for Daphnia

“…Therefore, measuring only juvenile growth rates could have revealed greater differences among diet treatments, but it was unfortunately not possible in our study. Overall, the growth rates of Daphnia in our experiments (II and III) on diets with varying proportion and content of tPOM were lower than in previous studies, where Daphnia were fed solely with phytoplankton for similar time periods (Hiltunen et al., ; Peltomaa et al., ). The reproductive output of Daphnia was slightly enhanced when they were fed aged tPOM compared to fresh tPOM, but only when the proportion of tPOM in the diet was very high (≥95%).…”

“…However, the mean number of neonates and eggs produced by Daphnia was very low (<6 per individual), even in the treatments with 25% phytoplankton. Previous studies have found that Daphnia typically produce > 20 eggs and neonates in 14 days when solely fed phytoplankton (excluding cyanobacteria) (Brett et al., ; Hiltunen et al., ; Peltomaa et al., ). Although our hypothesis 2 was supported since Daphnia performance was slightly enhanced with aging of tPOM, our results indicate that even tPOM aged for a long time is a much inferior food source for Daphnia compared to algae.…”

“…However, this conditioning effect has not been studied in pelagic systems, where a more favourable surface area to volume ratio could enhance microbial processing. The biochemical composition of food, particularly its lipid composition, is an important factor determining its nutritional quality for herbivorous zooplankton and influences both growth and reproduction (Brett & Müller‐Navarra, ; Brett et al., ; Martin‐Creuzburg & Von Elert, ; Peltomaa, Aalto, Vuorio, & Taipale, ; Taipale et al., ). Phytoplankton primary production is the main source of carbohydrates, lipids, and proteins for secondary consumers.…”

Simulated eutrophication and browning alters zooplankton nutritional quality and determines juvenile fish growth and survival

“…As bacteria contains equal EAAs to phytoplankton while tPOM is distinctly lacking in overall abundance of EAA, the EAA limitation of zooplankton in dystrophic lakes likely happens when zooplankton are forced to feed on terrestrial organic matter. These low nutritional quality periods can lead to decreased growth and reproduction of zooplankton due to the lack of both EAA and ω‐3 PUFA (Kleppel, Burkart, & Houchin, ; Peltomaa et al., ).…”

“…Intermediate quality diet for zooplankton includes all essential biomolecules, but in low amounts, whereas poor quality diet lacks some of the essential biomolecules. Daphnia can achieve high somatic growth with high or intermediate quality diet, whereas high reproduction rate requires high amounts of all essential biomolecules (Peltomaa et al., ). Generally, cryptophytes, synurophytes, and diatoms are high‐quality food for Daphnia , whereas green algae are intermediate quality due to the lack of EPA (Peltomaa et al., ).…”