Benthic Fauna

Benthic Fauna Extremely diverse group of organisms Many different groups, difficult to generalize about patterns

Distribution and Abundance Limited by few general characteristics Food availability Type of substrate Few phys/chem factors, especially dissolved oxygen

Littoral vs. Profundal Benthic animals living in littoral region more varied than those in profundal region Reflection of: 1) abundance of microhabitats 2) less stressful living conditions

Littoral Benthos Protozoans, sponges, coelenterates, rotifers, nematodes, bryozoans, decapods, ostracods, cladocerans, copepods, bivalves, snails, insects, leeches

Littoral Benthos Microbenthos - very tiny Vastly outnumber macrobenthos, and may contribute up to 50% of benthic production

Littoral Benthos Seldom food limited Proximity to phytoplankton, macrophytes

Sublittoral Benthos Boundary between littoral & profundal Species diversity drops off sharply Mussels, ostracods, copepods, cladocerans from littoral - few typical dwellers

Profundal Benthos Very poor diversity Oxygen limited Other stressors: Colder Lower pH Higher CO2, CH4, organics, P, NH3 Few can survive under these conditions

Profundal Benthos Profundal benthos in eutrophic lakes resemble those of grossly polluted systems Low diversity, monotony - great number of individuals, but only 1 or 2 species represented

Typical Profundal Assemblage Chironomus midge larvae Hemoglobin picks up limited oxygen May also be able to use anaerobic respiration & excrete products of this process

Typical Profundal Assemblage Oligochaete worms: Tubifex, Limnodrilus Bury heads in organic sediments, wave tails with gills Can develop huge populations (10,000+/m2) Preyed on heavily by predaceous Chironomus

Typical Profundal Assemblage Fingernail clams in genus Pisidium Become dormant during anaerobic periods May also inhabit temporary ponds

Typical Profundal Assemblage Phantom midge larvae - Chaoborus Temporary occupants - spend day on sediments, migrate into water column at night to prey on zooplankton Not very tolerant of anoxia

Typical Profundal Assemblage Also several microscopic forms that tolerate low oxygen Some protozoan ciliates and flagellates, some nematodes

General Standing Crop Most lakes have profundal benthos that averages ~5 g wet weight/m2 ~1/2 g dry weight/m2

Less Productive Lakes Profundal benthos more diverse in less productive (oligotrophic) lakes Major reason: oxygenated sediment-water interface

Less Productive Lakes More species of midge larvae, oligochaetes, immature insects like mayflies Hexagenia - burrowing mayfly

Less Productive Lakes Also more crustaceans like the amphipod Pontoporeia Makes vertical migrations up to metalimnion at night (doesn’t eat zooplankton)

General Benthos Pattern Diverse group in heterogenous, oxygenated littoral zone Less diversity in more homogeneous profundal zone (less in more productive lakes)

Maxima of Abundance Two maxima - one in littoral - one in profundal biomass depth

Maxima of Abundance As systems become more productive, zone of max. production shifts from littoral to profundal, then declines in profundal biomass depth Midges replaced by oligochaetes

Seasonal Abundance Patterns Lowest in summer (especially in insect-dominated communities) Emergence of adults, high predation Maximum densities and growth typically in autumn and winter in temperate zone

Predation by Fish Predation can drastically reduce invertebrate standing crop May be >50% of populations in some littoral areas Predation losses in profundal areas generally much lower

Predation by Fish Despite intense predation pressure, benthos dynamics and production mostly controlled by food supply