Diatoms (Benthic freshwater samples)

Algae are key primary producers in Arctic freshwaters, and benthic samples include diatoms and a number of classes of other algal groups. The Benthic Algae FEC describes primary producers that mainly occur in aquatic habitats, and includes both diatoms (class Bacillariophyceae) and soft algae (non-diatoms such as cyanobacteria, green algae, etc.). Algae from benthic samples are a heterogeneous group, ranging in size from micrometerlong small diatoms, cyanobacteria, or green algae to meter-long filamentous green algae. In benthic habitats, algae generally form a slippery, millimeter thick biofilm or thicker algal mats of around a centimeter. Benthic algae are important in food webs, and are often the main primary producers in open and shallow river stretches or lakes in the Arctic (Lento et al, 2019).

The benthic algal community is taxonomically rich, forms the base of the aquatic food web and has been shown to respond to changes in water quality. It is relatively simple to add benthic algal sampling to a monitoring program, as sample collection and processing are uncomplicated, processing costs are relatively low, and benthic algae are ubiquitous. In addition, there are established protocols for sampling and analyzing diatoms and chlorophyll a in several countries.

To enable the comparison of benthic algal samples throughout the Arctic, and at the same time to ensure that both biodiversity and biomass/biovolume are captured even with restricted resources, sampling in Arctic rivers must follow the general guidelines:

• Taxonomic identification of samples is essential for the assessment of change in community indices.
• Samples should be collected from the top of rocks/stones if available, as this is the focal substrate in most existing protocols. If rocks or stones are not present, core samples can be collected from soft substrates. Soft substrate sampling methods should be noted as comparability between streams may be affected.
• Sampling should occur annually, with primary data collection in late summer or early fall to fitpeak abundance and diversity, and to be consistent with historical sampling (Culp et al, 2012).

The sampling approach for benthic algae in Arctic lakes should follow these guidelines:

• Sampling of benthic communities should occur annually. Benthic algal surveys should be completed during the most ecologically relevant season, generally when biological diversity is highest. For many Arctic lakes this means August/September or one month before ice cover, when the majority of taxa will be present and the biomass is highest. To reduce costs and increase “data value”, sampling should be linked with other programs (zooplankton, water chemistry etc.). The use of a common field protocol describing sampling depths, bottom substrata, etc. is recommended.
• To ensure adequate spatial placement of samples, benthic algae should ideally be collected from a known area of all major types of bottom material (sand, pebbles and stones) ranging from the shallow littoral (0.5 m) to the deepest point of the lake (when possible), with multiple samples collected along this depth gradient. However, in continuous monitoring this sampling design may be too expensive, and sampling should minimally be conducted by collecting benthic algae from the littoral zone (Culp et al 2012).