Copepod

Copepods are tiny freshwater crustaceans (0.5-2 mm) and one of the most ubiquitous animals in ponds, puddles, and aquariums — they hitchhike in on live plants and in water from other tanks. The parameters in this generic copepod are calibrated to cyclopoid copepods (the type hobbyists are most likely to encounter), which hunt with a fundamentally different feeding strategy from Daphnia. Instead of filter-feeding, they are raptorial hunters -- they actively detect and capture individual prey items. This makes them less efficient at consuming bulk phytoplankton but better at targeting specific food sources. Copepods are true omnivores in the model: they eat planktonic algae (with low access -- they can only catch small fragments), periphyton, bacteria, suspended detritus, and importantly, ciliates. This predation on ciliates completes the microbial loop, channeling energy from dissolved organics through bacteria through ciliates into copepod biomass. Copepods reproduce sexually, which makes their population growth slower than the parthenogenetic Daphnia. They are generally hardier than Daphnia -- they tolerate wider temperature ranges (stressed only below 2 degrees or above 28 degrees Celsius), cope better with hypoxia, survive longer without food thanks to lipid reserves, and have a lower base mortality rate (about 1.5% per day). Instead of crowding mortality, copepods have density-dependent cannibalism -- adults consume their own nauplii (larvae) and early juveniles at high population densities, which similarly prevents runaway population growth. Their fecal pellets are smaller and more prone to breakup than Daphnia's (35% stays suspended versus 25% for Daphnia). Copepods are more active at night than Daphnia (80% versus 70% of daytime activity). They respect surface-specific refugia when grazing on periphyton, bacteria, and microzooplankton -- protected surfaces contribute less food even though biomass is present there. For periphyton, access is calculated per surface. For bacteria and microzooplankton, effective access is a surface-area-weighted average across all surfaces in the scenario.

Life stages and resting cysts

Copepods in EcoSym are tracked in three pools per instance: a juvenile (naupliar/copepodite-equivalent) pool, an adult pool, and a dormant 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 resting eggs (calanoid diapausing eggs) or resting copepodites (cyclopoid encystment) instead of as active juveniles. The two strategies are mechanism-equivalent at ecosystem-flux scale (Hairston 1987, 1996; Frisch 2002; Hansen 1998).

The maturation lag is calculated using Belehrádek's equation D = a × (T − α)^b, with a ≈ 26600, α = −10 °C, b = −2.05 fitted to calanoid egg→adult development (Munro 1974; Hart 1990). At 20 °C this gives ~25 days, considerably longer than Daphnia. The longer lag explains why copepod populations respond more slowly to food availability and recover more slowly from population crashes — both of which match field observations of cyclopoid and calanoid annual cycles.

The dormant pool is metabolically inert and bypassed by every active-pool mortality kernel — Cu²⁺ toxicity, hypoxia, NH₃, NO₂⁻, temperature stress, pH stress — and is exempt from the extinction-threshold check. This is what allows a copepod population that has lost its entire active stage to a hypoxic summer episode or a copper pulse to persist in sediment cysts and re-seed the system when conditions improve. Allocation cues mirror Daphnia (food limitation, density, low temperature, short photoperiod) but the cyclopoid summer-encystment behaviour is reflected in a slightly higher dormant-fraction ceiling (50% vs 40% for Daphnia, with a 4% constitutive background rate). Hatching is gated by rewarming above 10 °C and Monod-saturated food availability.