6.4 Population dynamics

Population growth and decline

Population growth and decline describe how zooplankton numbers change over time in lakes and other freshwater systems. These changes result from the interplay between organisms and their environment, and tracking them is central to understanding lake ecosystem health.

Factors affecting population size

Both biotic and abiotic factors drive zooplankton population size.

• Biotic factors include competition for food (especially phytoplankton), predation by fish and invertebrates like Chaoborus larvae, and disease
• Abiotic factors include water temperature, pH, dissolved oxygen, nutrient concentrations, and light availability

These factors don't act in isolation. For example, nutrient loading can trigger algal blooms that initially boost food supply for herbivorous zooplankton like Daphnia, but the subsequent oxygen depletion from bloom decomposition can then crash those same populations.

Carrying capacity of ecosystems

Carrying capacity ($K$) is the maximum population size a lake or habitat can sustain given available resources. As a zooplankton population approaches $K$, individuals compete more intensely for food and space, which slows growth. If a population temporarily overshoots $K$, resource depletion and increased mortality can cause a sharp decline. This boom-and-bust pattern is common in zooplankton communities during seasonal transitions.

Density-dependent vs. density-independent factors

• Density-dependent factors have effects that intensify as population density increases. Examples: competition for phytoplankton, predation pressure from planktivorous fish, and disease transmission among crowded zooplankton
• Density-independent factors affect populations regardless of density. Examples: sudden temperature drops, storm-driven mixing events, and pesticide runoff

In practice, zooplankton populations are shaped by both types simultaneously. A Daphnia population might be regulated by fish predation (density-dependent) during summer, then decimated by autumn lake turnover (density-independent).

Population interactions

Interactions between species shape community structure in the plankton. Who eats whom, who competes with whom, and who benefits from whom all determine which zooplankton dominate a lake at any given time.

Competition for resources

Competition occurs when organisms vie for the same limited resource.

• Interspecific competition happens between different species. For instance, Daphnia and Bosmina may compete for the same size fraction of algae.
• Intraspecific competition occurs within a single species and often intensifies at high densities.

Competition can lead to resource partitioning, where species divide up food or habitat to reduce overlap, or to competitive exclusion, where one species eliminates another from a habitat entirely. In many lakes, you'll see niche differentiation where different cladoceran species specialize on different algal size classes.

Predator-prey relationships

Predation is one of the strongest forces structuring zooplankton communities. Planktivorous fish (like young perch or alewife) selectively consume larger, more visible zooplankton, while invertebrate predators like Chaoborus and cyclopoid copepods target smaller prey.

Predator-prey dynamics often produce cyclical population fluctuations: prey numbers rise, predators respond by increasing, prey then decline from heavy predation, and predators follow. This lag between predator and prey peaks is a hallmark of these interactions and is well-documented in lake zooplankton time series.

Symbiotic relationships

Symbiotic relationships involve close, sustained interactions between species. While less prominent in zooplankton ecology than predation or competition, they still matter.

• Mutualism: both species benefit. Some zooplankton harbor endosymbiotic algae that provide supplemental nutrition.
• Commensalism: one species benefits, the other is unaffected. Certain rotifers attach to larger crustacean zooplankton for transport without harming the host.
• Parasitism: one species benefits at the other's expense. Microsporidian parasites infecting Daphnia populations can reduce host fecundity and alter population dynamics significantly.

Population structure

Population structure describes how a population is organized by age, sex, and genetic makeup. It tells you a lot about whether a population is growing, stable, or declining.

Age structure and distribution

Age structure is the proportion of individuals in different life stages: juveniles (neonates), reproductive adults, and post-reproductive individuals. In zooplankton like Daphnia, you can assess age structure by measuring body size distributions in net samples.

• A population dominated by juveniles is likely growing rapidly.
• A population skewed toward older individuals may be experiencing recruitment failure.
• A stable age distribution has constant proportions in each age class over time, indicating steady-state conditions.

Sex ratios and mating systems

Most cladocerans (like Daphnia) reproduce by cyclical parthenogenesis: females dominate the population and reproduce asexually under favorable conditions. Males appear primarily when environmental stress triggers sexual reproduction, which produces resting eggs (ephippia) that can survive harsh conditions.

A sudden shift in sex ratio toward more males signals that the population is responding to deteriorating conditions. In copepods, sex ratios tend to be more balanced, and mating is obligately sexual.

Genetic diversity within populations

Genetic diversity is the variety of alleles and genotypes in a population. Higher diversity gives a population more raw material to adapt to changing lake conditions, such as shifts in temperature, food quality, or predation regime.

Genetic diversity can decline through:

• Population bottlenecks (drastic reductions in population size)
• Inbreeding (mating among close relatives in small, isolated populations)
• Genetic drift (random changes in allele frequencies, especially in small populations)

Low genetic diversity makes zooplankton populations more vulnerable to disease outbreaks and environmental change.

Population regulation mechanisms

Four processes govern changes in population size: births, deaths, immigration, and emigration. The balance among these determines whether a population grows, shrinks, or stays stable.

Birth rates and fecundity

Birth rate is the number of new individuals produced per unit time. Fecundity is the reproductive potential, often measured as offspring per female per clutch.

In Daphnia, fecundity is strongly tied to food availability and temperature. A well-fed Daphnia at 20°C might produce 10-20 neonates per clutch every 3-4 days, while a food-limited individual might produce only 2-3. Tracking clutch size in field samples is a practical way to assess population health.

Death rates and mortality

Death rate is the number of individuals lost per unit time. Major sources of zooplankton mortality include fish predation, invertebrate predation, starvation, parasitism, and physiological stress from low oxygen or extreme temperatures.

Age-specific mortality is particularly informative. If juvenile mortality is high, the population may struggle to replace itself even if adults survive well. Constructing life tables for zooplankton populations helps identify which life stages are most vulnerable.

Immigration and emigration

• Immigration: individuals entering a population from elsewhere
• Emigration: individuals leaving a population

In zooplankton, these processes often take the form of diel vertical migration (DVM), where animals move between depth strata, or horizontal transport between lake basins by currents. Dispersal between lakes occurs mainly through resting eggs carried by wind, water flow, or animal vectors (like waterfowl). Immigration can introduce new genetic material, while emigration can relieve local density-dependent pressures.

Population dynamics models

Mathematical models help predict how zooplankton populations change over time. They simplify reality to capture key dynamics and are useful for testing hypotheses and guiding management.

Exponential growth model

The exponential model assumes unlimited resources and no density effects:

$N_t = N_0 e^{rt}$

where $N_t$ is population size at time $t$, $N_0$ is initial population size, $r$ is the intrinsic rate of natural increase, and $e$ is the base of the natural logarithm.

This model produces a J-shaped growth curve. It can approximate zooplankton population growth during brief windows of abundant food and low predation (such as the early spring bloom), but no population grows exponentially for long.

Logistic growth model

The logistic model adds carrying capacity ($K$) to account for resource limitation:

$\frac{dN}{dt} = rN \left(1 - \frac{N}{K}\right)$

where $N$ is population size, $r$ is the intrinsic growth rate, and $K$ is carrying capacity.

This produces an S-shaped (sigmoidal) curve: rapid initial growth that slows as the population approaches $K$ and stabilizes. The term $(1 - N/K)$ acts as a brake on growth. When $N$ is small relative to $K$, growth is nearly exponential. As $N$ nears $K$, growth approaches zero.

Lotka-Volterra predator-prey model

This model captures the coupled dynamics of a predator and its prey using two differential equations:

• Prey: $\frac{dN}{dt} = rN - aNP$
• Predator: $\frac{dP}{dt} = baNP - mP$

where $N$ = prey population, $P$ = predator population, $r$ = prey growth rate, $a$ = attack rate (predation coefficient), $b$ = conversion efficiency (how effectively consumed prey translates into new predators), and $m$ = predator mortality rate.

The model predicts oscillating populations: prey increase, predators follow with a time lag, prey crash under heavy predation, then predators decline from starvation. These out-of-phase cycles are a reasonable first approximation of fish-zooplankton or invertebrate predator-zooplankton dynamics in lakes, though real systems are more complex due to multiple prey species, refugia, and environmental variability.

Human impacts on aquatic populations

Human activities are among the most powerful forces shaping zooplankton populations in lakes and reservoirs. These impacts often cascade through the food web.

Overfishing and exploitation

When planktivorous fish are overharvested, predation pressure on large zooplankton drops. This can trigger a trophic cascade: large Daphnia proliferate, graze down phytoplankton, and water clarity improves. Conversely, stocking too many planktivorous fish suppresses large zooplankton, allowing algae to bloom.

Sustainable management practices (catch limits, size restrictions, gear regulations) help maintain balanced food webs. This principle underlies biomanipulation, a lake management technique that adjusts fish populations to control algal blooms through zooplankton grazing.

Habitat destruction and fragmentation

• Habitat destruction: activities like dredging, shoreline development, and wetland filling eliminate littoral habitats that many zooplankton use for refuge and reproduction
• Habitat fragmentation: damming rivers or isolating lake basins reduces connectivity, limiting dispersal and gene flow between zooplankton populations

Preserving and restoring littoral zones, wetlands, and hydrological connectivity is critical for maintaining diverse zooplankton communities.

Invasive species introductions

Invasive species are non-native organisms that establish, spread, and cause ecological harm. Major vectors include ballast water discharge, aquaculture, and the aquarium trade.

Notable examples in freshwater systems:

• Zebra mussels (Dreissena polymorpha) filter massive volumes of water, depleting phytoplankton and directly competing with filter-feeding zooplankton
• Bythotrephes (spiny water flea) is an invasive predatory cladoceran that preys on native zooplankton and can restructure entire zooplankton communities

Prevention through ballast water treatment, public education, and early detection programs is far more effective than attempting to control established invasive populations.

Conservation and management strategies

Protecting zooplankton populations and the lakes they inhabit requires combining scientific monitoring with sound policy and community involvement.

Population monitoring techniques

Regular monitoring tracks abundance, species composition, and demographic characteristics over time. Common techniques include:

• Vertical net tows and horizontal tows to sample zooplankton density and community composition
• Mark-recapture studies (more common for fish, but applicable to some larger crustaceans)
• Acoustic monitoring to track diel vertical migration patterns
• Environmental DNA (eDNA) analysis to detect species presence, including rare or invasive taxa

Long-term datasets are especially valuable for detecting trends, evaluating management actions, and distinguishing natural variability from human-caused change.

Sustainable harvesting practices

While zooplankton themselves are rarely harvested directly, sustainable fisheries management has cascading effects on zooplankton communities. Key practices include:

• Setting catch limits based on stock assessments
• Implementing size and age restrictions to protect spawning adults
• Using selective gear to minimize bycatch and habitat damage
• Ecosystem-based fisheries management, which considers how harvesting one species affects the rest of the food web, including zooplankton

Habitat restoration and protection

Restoring degraded lake and wetland habitats improves conditions for zooplankton and the broader aquatic community. Strategies include:

• Re-establishing littoral vegetation and riparian buffers
• Reducing nutrient inputs to reverse eutrophication
• Creating protected areas or no-take zones in critical habitats
• Reconnecting fragmented water bodies to restore dispersal corridors

Engaging local communities in restoration efforts builds long-term stewardship and increases the likelihood that conservation gains persist.