Daphnia

Daphnia (water fleas) are small planktonic crustaceans (0.2-5 mm) that filter-feed by sweeping water through their thoracic appendages. They are non-selective filter feeders, consuming particles in the 1-50 micrometer size range -- this means they eat planktonic algae, surface-attached algae (with limited access), periphyton, bacteria, and suspended detritus. Daphnia reproduce primarily through parthenogenesis (asexual cloning), which allows extremely rapid population growth under good conditions. They have the highest maximum ingestion rate of the two grazers (about 1.4 times body carbon per day). Daphnia are strictly freshwater (optimal 0.5 PSU, lethal above 10 PSU) and prefer cool to moderate temperatures (stressed above 25 degrees Celsius, lethal above 30 degrees). They are sensitive to ammonia toxicity, with stress beginning at quite low unionized ammonia concentrations. The model also tracks density-dependent crowding mortality: at high Daphnia densities, increased competition, metabolite accumulation, and disease transmission cause extra mortality, which prevents runaway population booms. Their fecal pellets are relatively large and dense, with 75% settling to the bottom. When Daphnia die, 90% of their biomass settles.

Life stages and resting eggs

Daphnia in EcoSym are tracked in three pools per instance: a juvenile pool, an adult pool, and a dormant (ephippial) pool. Only adults reproduce; juveniles mature into adults after a temperature-dependent developmental lag, and adults under stress route part of their offspring into the dormant pool as ephippia (sexual diapausing eggs) instead of into the juvenile pool.

The maturation lag is the dominant control on real Daphnia–phytoplankton oscillation periods. In a single-pool model where all biomass reproduces instantaneously, classic predator-prey theory predicts cycles of 5–15 days; with the maturation lag, real lakes and mesocosms run at the empirical 20–60 day periods (McCauley & Murdoch 1987; McCauley et al. 1999, 2008). The lag is calculated using Belehrádek's equation D = a × (T − α)^b, with a ≈ 10650, α = −10 °C, b = −2.05 fitted to D. magna egg→primipara development across the literature (Bottrell 1975, 1976; Goss & Bunting 1983). At 20 °C this gives ~10 days; at 25 °C ~7 days; at 15 °C ~16 days.

The dormant pool is metabolically inert: it does not feed, respire, or grow, and is bypassed by every active-pool mortality kernel — Cu²⁺ toxicity, hypoxia, NH₃, NO₂⁻, temperature stress, pH stress. It is also exempt from the extinction-threshold check that disables active growth at very low biomass. This means a Daphnia population that loses its entire active stage to an acute event (anoxia, copper pulse, temperature shock, drying) can survive in the egg bank and re-seed the system when conditions improve, mirroring the real-world resilience that makes ephippia recoverable from sediment cores up to centuries old (Cáceres 1998; Hairston et al. 1995).

Allocation between subitaneous (juvenile) and ephippial (dormant) offspring is gated by four cues averaged together — food limitation, crowding, low temperature, and short photoperiod — with a 3% constitutive background rate that builds an egg bank even under good conditions (Stross & Hill 1965; Carvalho 1987; Hobæk & Larsson 1990). Hatching from the dormant pool back into the juvenile pool is gated by rewarming above 10 °C and Monod-saturated food availability (De Meester & De Jager 1993; Cáceres 1998).