Quick Dips

Little Plankton Have Lesser Plankton...

The development of a new form of microscopy which allows researchers to easily identify and count very small organisms has led to the discovery of the importance of microbial organisms in the food webs in the open ocean surrounding New Zealand and a rethink of the conventional food chain.

Natural bacteria populations have been shown to range from 2.5-30 million cells per teaspoon of seawater in the waters surrounding New Zealand.

The bacteria are very small organisms, 0.1-1.0mm in diameter (a human hair is approximately 30mm in diameter), which gain their energy from organic compounds, such as glycolate, which are dissolved in the water. These compounds occur naturally in the ocean, with the main sources being direct release from phytoplankton (planktonic algae) cells, or indirectly via zooplankton (planktonic animals) either as a result of sloppy feeding (when cells break and contents leak into the water) or from the dissolving of material from zooplankton faecal material.

Recent analysis of samples collected by NIWA have shown that at half the sites sampled, bacteria are the dominant planktonic biomass, particularly in regions where overall planktonic biomass is low.

The use of high resolution epifluorescent microscopy has led to the discovery that picophytoplankton (algae 0.2-2.0mm in diameter) are very important in the ocean surrounding New Zealand. They contribute 40-80% of the plant production and up to 80% of the plant biomass in the ocean. These organisms can occur at concentrations of up to 500,000 cells per teaspoon of water.

At all the sites sampled, planktonic biomass was dominated by the combined biomass of the bacteria and the picophytoplankton, suggesting that these small organisms play a very significant role in food web structure and dynamics. In fact the very high numbers and contribution to biomass of these two types of very small organisms has led to a rethinking of the classical pyramid-shaped food web structure in which phytoplankton larger than 20mm are directly consumed by zooplankton larger than 200mm, which then form a food source for fish.

The bacteria and picophytoplankton are so small that they cannot be consumed directly by the zooplankton, so they need to be eaten and packaged into larger parcels which can then be eaten by the zooplankton. The organisms which eat the bacteria and picophytoplankton are microzooplankton, organisms less than 200mm. This means that there is at least one more step in the food web than previously known, particularly in water where bacteria dominate the plankton biomass.

The length of the food web has a direct relationship to the amount of energy used to produce a fish -- the longer the food web, the more energy is required. This also has implications for the stability and resilience of the ecosystem. The importance of bacteria and picophytoplanktons mean that understanding the microbial food web processes is a key factor in understanding the ecology of the oceans.

Dr Julie Hall works for NIWA