Effects of zooplankton dynamics on epilimnetic phosphorus loss

Abstract

1. Our aim was to analyse the impact of zooplankton dynamics on the relative importance of two mechanisms contributing to the loss of phosphorus (P) from the epilimnion of stratified lakes: net population incorporation into zooplankton biomass and P sedimentation. 2. We established enclosures without Daphnia (control), with a growing Daphnia population (treatment D) and with a high, stable Daphnia population (treatment D+). The P incorporated in zooplankton biomass and sedimented was measured at short intervals over a period of 17 days. 3. In both Daphnia treatments, sedimentation increased and the P content of sedimented matter was higher than in the control and highest in D+. The P loss by sedimentation between day 3 and 17 was generally high (>25%, >1.8% day )1 ) with particularly high values in D+ (60%, 4.3% day )1 ). Phosphorus sedimentation was higher in the zooplankton treatments, although the contribution of exuviae and dead Daphnia was minor. Faecal material was probably a major component of sedimentation. 4. By contrast, the amount of P in zooplankton (mainly Daphnia) biomass increased in D but remained constant in D+. Phosphorus loss owing to net population incorporation was generally low and ranged up to 6.5% (0.5% day )1 ) in treatment D. A positive relationship between Daphnia dry mass and P sedimentation, as well as P incorporation, was found. 5. Sedimentation is evidently an important cause of P loss from the epilimnion where Daphnia is abundant. By contrast, P incorporation into Daphnia biomass may only become an important loss factor when the population is growing.

Brine shrimp grazing and fecal production increase sedimentation to the deep brine layer (monimolimnion) of Great Salt Lake, Utah

“…Moreover, the results of our study also revealed that the stoichiometric ratio of C:N in the tissues of Artemia decreased during the second experiment (this ratio was not assessed in the first experiment). The results are consistent with those of earlier studies about the effect of zooplankton on the stoichiometry of sediments in marine and freshwater environments (e.g., Pitsch et al, 2012) and with the stoichiometric theory, which states that the deficient elements (in this case N) are captured in the biomass and the excess elements are excreted or defecated. The analysis of the stable isotopes of nitrogen and carbon revealed other interesting results.…”

“…Many studies in both marine and freshwater have found that zooplankton decreases the vertical flux of nutrients and organic matter (e.g., Bathmann et al, 1987;Ayukai & Hattori, 1992;Sarnelle, 1999;Yoshimizu & Urabe, 2002), which has been attributed to a high grazing rate of zooplankton on fecal material, the production of bacteria associated with zooplankton activity, and a high primary production. Other studies have reported an opposite relationship (Honjo & Roman, 1978;Bloesch & Bürgi, 1989;Andreassen et al, 1996;Pilati & Wurtsbaugh, 2003;Pitsch et al, 2012), which is mainly attributed to the presence of a great number of large zooplankton taxa, which produce fecal pellets that sink quickly and resist decomposition (e.g., copepods and euphausiids, Wotton & Malmqvist, 2001). Surprisingly, there have been only a few studies on the effects of zooplankton on the vertical particulate flux in saline lakes (Jellison et al, 1993, Jellison & Melack, 2001) and ponds (Bruce & Imberger, 2009), despite the fact that salinity (density) might have a large (direct and indirect) influence on sedimentation rates.…”

“…Sedimentation rates are also influenced by the size of phytoplankton cells (Dagg et al, 2014), the strength of thermal stratification (Alldredge et al, 1987;Evans et al, 1998), and many other factors (Rosa, 1985;Eadie et al, 1989;Evans et al, 1998;Hannides et al, 2009). Pitsch et al (2012) also demonstrated that the effect of zooplankton on the nutrient loss from a lake's epilimnion is much stronger through sedimentation than through net incorporation into zooplankton biomass. Many studies in both marine and freshwater have found that zooplankton decreases the vertical flux of nutrients and organic matter (e.g., Bathmann et al, 1987;Ayukai & Hattori, 1992;Sarnelle, 1999;Yoshimizu & Urabe, 2002), which has been attributed to a high grazing rate of zooplankton on fecal material, the production of bacteria associated with zooplankton activity, and a high primary production.…”