Food Web Structure in the Recently Flooded Sep Reservoir as Inferred From Phytoplankton Population Dynamics and Living Microbial Biomass

Abstract

Phytoplankton dynamics, bacterial standing stocks and living microbial biomass (derived from ATP measurements, 0.7-200 mm size class) were examined in 1996 in the newly flooded (1995) Sep Reservoir ('Massif Central,' France), for evidence of the importance of the microbial food web relative to the traditional food chain. Phosphate concentrations were low, N:P ratios were high, and phosphate losses converted into carbon accounted for <50% of phytoplankton biomass and production, indicating that P was limiting phytoplankton development during the study. The observed low availability of P contrasts with the high release of "directly" assimilable P often reported in newly flooded reservoirs, suggesting that factors determining nutrient dynamics in such ecosystems are complex. The phosphate availability, but also the water column stability, seemed to be among the major factors determining phytoplankton dynamics, as (i) large-size phytoplankton species were prominent during the period of increasing water column stability, whereas small-size species dominated phytoplankton assemblages during the period of decreasing stability, and (ii) a Dinobryon divergens bloom occurred during a period when inorganic P was undetectable, coinciding with the lowest values of bacterial standing stocks. Indication of grazing limitation of bacterial populations by the mixotrophic chrysophyte D. divergens (in late spring) and by other potential grazers (mainly rotifers in summer) seemed to be confirmed by the Model II or functional slopes of the bacterial vs phytoplankton regressions, which were always <0.63. Phytoplankton biomass was not correlated with phosphorus sources and its contribution was remarkably low relative to the living microbial biomass which, in contrast, was positively correlated with total phosphorus in summer. We conclude that planktonic microheterotrophs are strongly implicated in the phosphorus dynamics in the Sep Reservoir, and thus support the idea that an important amount of matter and energy flows through the "microbial loop" and food web, shortly after the flooding of a reservoir.

Functional responses of prokaryotes and viruses to grazer effects and nutrient additions in freshwater microcosms

“…Inorganic nutrient concentrations were low, except for nitrate (0.38 mg l À1 ). The very low concentration of dissolved inorganic phosphate (P, 0.002 mg l À1 ) compared to inorganic nitrogen and the related high N:P atomic ratios (4200) indicated severe P limiting conditions, typical of the Sep Reservoir during summer (Tadonléké et al, 2002). Chlorophyll a pigment was about half the median value known from the annual range (that is, 1-43 mg l À1 ) recorded in the euphotic layer during the study year .…”

“…The N:P ratio in the experimental samples (223:1) was very high, characteristic of P limiting conditions in the Sep Reservoir during the summer (Tadonléké et al, 2002). Limitations of prokaryotic growth and activity in Sep Reservoir especially by P has always been inferred from the measured concentration of potential limiting substrates rather than by nutrient bioassay approach.…”

“…Each set of bottles (both o0.8 and o200 mm fractions) received organic and inorganic nutrients, individually and in combinations, including carbon (glucose 125 mM C), nitrate (NaNO 3 25 mM N) and phosphate (NaH 2 PO 4 5 mM P) in four different treatments, namely, C, P, NP and CNP. Nutrients were added to reach up to their maximum concentrations known from the annual range recorded previously in the euphotic layer of the Sep Reservoir (Tadonléké et al, 2002;Pradeep Ram et al, 2005). One set of bottles which received no nutrient addition served as unaltered control (U).…”

Grazer effects on prokaryotes and viruses in a freshwater microcosm experiment

“…The very low concentration of dissolved inorganic phosphate and the related high N:P atomic ratio (ca. 35:1) indeed indicated P-limiting conditions, typical of the Sep Reservoir during summer (Tadonléké et al 2000(Tadonléké et al , 2002. In terms of the biological environment, the experimental samples contained about 10 µg l -1 of chlorophyll a pigment, which is at the lower end of the annual range (i.e.…”

“…Our experiment was carried out during a clearwater phase, following the apparent exhaustion of phosphorus in the reservoir (Pradeep Ram et al 2005). The N:P ratio in the experimental samples was very high, characteristic of the typically severe P-limiting conditions in the Sep Reservoir during summer (Tadonléké et al 2000(Tadonléké et al , 2002. In our unfiltered treatment, prokaryotic growth was likely forced by regenerated nutrients and labile organic substrates from grazer activity.…”