Phosphorus-limited Algae Research Articles

Sink or swim? Vertical movement and nutrient storage in phytoplankton

A simulation model of vertically migrating phytoplankton is presented, using a Lagrangian, individual-based computational approach. Algal cells acquire and store nutrient at the bottom of the habitat, using stored nutrient to grow while in shallower waters. Stored nutrient also governs movement: cells sink when their nutrient quota falls below a threshold; otherwise they rise (or at least sink more slowly). Although the bottom of the habitat provides the growth-limiting nutrient, it also entails a risk of mortality. For a parameter set representing phosphorus-limited algae with a fixed nutrient storage capacity, neither continual sinking nor continual rising are optimal strategies. Instead, an adaptive dynamics approach suggests there is an optimal movement strategy in which cells rise when their storage capacity is partially filled, and otherwise sink. When the movement strategy is fixed in such a way and storage capacity is free to evolve, storage capacity approaches an optimal value several times higher than the minimal quota permitting population growth. Vertical movement and nutrient storage affect the vertical distribution of total nutrient. When cells always sink, total nutrient declines exponentially from the nutrient source at the bottom to a surface minimum. When cells always rise, there is a peak of total nutrient at the bottom, and another at the surface, with a minimum between. When cells move optimally, the vertical distribution of total nutrient can be close to uniform, or have a peak at mid-depth.

Does the nutrient stoichiometry of primary producers affect the secondary consumer Pleurobrachia pileus?

We investigated whether phosphorus limitations of primary producers propagate upwards through the food web, not only to the primary consumer level but also onto the secondary consumers’ level. A tri-trophic food chain was used to assess the effects of phosphorus-limited phytoplankton (the cryptophyte Rhodomonas salina) on herbivorous zooplankters (the copepod Acartia tonsa) and finally on zooplanktivores (the ctenophore Pleurobrachia pileus). The algae were cultured in phosphorus-replete and phosphorus-limited media before being fed to two groups of copepods. The copepods in turn were fed to the top predator, P. pileus, in a mixture resulting in a phosphorus-gradient, ranging from copepods having received only phosphorus-replete algae to copepods reared solely on phosphorus-limited algae. The C:P ratio of the algae varied significantly between the two treatments, resulting in higher C:P ratios for those copepods feeding on phosphorus-limited algae, albeit with a significance of 0.07. The differences in the feeding environment of the copepods could be followed to Pleurobrachia pileus. Contrary to our expectations, we found that phosphorus-limited copepods represented a higher quality food source for P. pileus, as shown by the better condition (expressed as nucleic acid content) of the ctenophore. This could possibly be explained by the rather high C:P ratios of ctenophores, their resulting low phosphorus demand and relative insensitivity to P deficiency. This might potentially be an additional explanation for the observed increasing abundances of gelatinous zooplankton in our increasingly phosphorus-limited coastal seas.

Gut passage of phosphorus-limited algae through Daphnia: do they take up nutrients in the process?

Nutrient-limited algae are known to be a food source of inferior quality for zooplankters. Three factors are thought to determine this poor quality: direct elemental limitations of the algae, biochemical limitations and an increased resistance to digestion because of an increase in cell wall thickness. Thus far, most studies have concentrated on the effect of the algae on the daphniids. It has recently been hypothesized, however, that while going through the digestive tract of herbivorous zooplankters the digestion resistant nutrient-limited algae might actually take-up nutrients, in a similar way as it has been described for gelatinous alga such as Sphaerocystis. In this study, we present results of different experiments investigating whether nutrient-limited algae are indeed more resistant to digestion, and whether nutrient-limited algae take-up the limiting nutrient in the guts of their predators. We observed that digestion resistance is not very important, and that it can only be observed at high food levels. As a result, we could not find any evidence for nutrient uptake of these algae when they pass through the daphniids. We did find that animals adapted to low-P environments have a higher incorporation efficiency for P, and conclude that digestion resistance in nutrient stressed algae is of very limited ecological relevance.

No Evidence for Adaptive Micro-Evolution to a Decrease in Phosphorus-Loading of a Daphnia Population Inhabiting A Pre-Alpine Lake

Many polluted lakes in Europe are being restored and phosphorus concentrations have dropped dramatically in these lakes. We studied the genetic structure of Daphnia galeata over the past 30 years in Lake Greifensee, Switzerland, a period during which the phosphorus concentration in the lake reduced dramatically. Distinct genotypes of D. galeata were hatched from diapausing eggs extracted from six different time horizons in dated sediments. We compared juvenile growth, size and egg production of D. galeata reared on high-P and low-P algae to investigate whether Daphnia have evolved to grow better on phosphorus-limited algae. Our results indicate that life histories of D. galeata differed significantly between both food types. We also found significant clone effects for size and egg number. But we found no significant interaction between the depth from which the clones were selected and food quality. This means that we found no evidence for adaptive micro-evolution in response to P reduction in the lake. We discuss our results in relation to other studies that found evidence for adaptive micro-evolution in comparable time frames.

Life history bottlenecks in Diaptomus clavipes induced by phosphorus‐limited algae

Considerable information has accumulated on food quality effects on certain cladocerans; meanwhile, other zooplankton have received comparatively little nutritional study. In particular, little is still known about nutrition of copepods with more complex ontogenetic changes. Recent studies have identified strong ontogenetic differences in phosphorus content of copepods, which have been found to be highest for late naupliar stages. Laboratory experiments were set up to examine the life cycle of a calanoid copepod relative to mineral P limitation on a pure algae diet. We hypothesized that P imbalance would cause nauplii particularly to be affected by algal nutrient status. Developmental rates, growth rates, and fecundity of Diaptomus clavipes under excess (1.0 mg C L−1) P‐deficient and P‐sufficient Scenedesmus obliquus were determined. We found strong differences between diets in survival to maturity: nauplii developed into copepodites and adults that successfully reproduced when fed the P‐sufficient algae but invariably died after molting into copepodite CII when reared on the P‐deficient algae. Differences in developmental rates were small for most life stages of nauplii for both food types but were substantial both for copepodite CI and particularly for CII that lived without molting up to 22 d on P‐deficient food before dying. Surprisingly, copepodites that fed on Pdeficient algae had similar or higher specific somatic growth rates than animals on P‐sufficient algae. Although these experiments demonstrate a strong ontogenetic component to copepod nutrition, and a heretofore undocumented dependence of copepod success on algal P content, the life history bottleneck occurs at a later stage than we hypothesized.

The nutritional quality of P‐limited algae for Daphnia

This paper details results of experiments intended to clarify the relative effects of mineral versus biochemical limitations of algae on the food quality for herbivorous zooplankton. Phosphorus limited algae (Scenedesmus) were fed to Daphnia magna and the somatic growth rates of the animals were measured. The animals showed an increase in growth rate when phosphorus (P) was added to P‐limited algae, especially when the animals were older. The addition of highly unsaturated fatty acids only resulted in an increase in growth rates when the algae had a C:P ratio of less then 300. The main conclusion of this work is that both mineral and biochemical limitations of their food play a role in the growth and population dynamics of zooplankters, but that mineral requirements need to be met first.