This study compared microbial populations in shallow (littoral) and deep (profundal) sediments of two English lakes with different nutrient levels. Using direct bacterial counts, ATP measurements, and enzyme activity assays, researchers found that profundal sediments consistently harbored higher microbial biomass and activity than littoral sediments in both a mesotrophic lake (Windermere South Basin) and a eutrophic lake (Blelham Tarn). The differences were more pronounced in the smaller, more productive lake. Profundal sediments contained higher concentrations of available organic substrates (carbohydrates, proteins, amino acids) in their interstitial water and had a greater proportion of smaller particles (<45 micrometers), which supported larger numbers of bacteria per gram dry weight. Littoral sediments showed greater diversity of benthic invertebrates, while ciliated protozoans were more abundant in profundal zones. The researchers attribute these differences to multiple factors: nutrient availability, sediment particle size distribution, and potentially animal grazing, though further experimental work is needed to determine their relative importance.

Key findings

• Microbial numbers, biomass, and activity were significantly higher in profundal (deep-water) sediments compared to littoral (shallow-water) sediments in both lakes studied
• Profundal sediments contained higher concentrations of available organic substrates (carbohydrates, proteins, amino acids) in interstitial water
• Profundal sediments had a greater proportion of small particles (<45 micrometers), which were colonized by more bacteria and showed higher metabolic activity per unit dry weight
• Ciliated protozoans were more abundant in profundal sediments, while larger benthic animals showed greater diversity in littoral zones
• Differences between littoral and profundal microbial communities were greater in the smaller, more productive eutrophic lake than in the larger mesotrophic lake