Overview Of The Arctic Microphytobenthos: Growth, Production, And Biodiversity
Arctic microphytobenthos growth
Across the Arctic littoral, the biomass of coastal benthic communities exhibits huge spatial and annual variations. One of the prevailing factors governing the occurrence and growth of the microphytobenthos is light availability. Other ecological factors add up for the regulation of the microphytobenthos across the Arctic seafloor. Arctic benthic microphytes are tightly dependent from regional oceanographic processes, atmospheric processes, seasonal nutrient admixtures, annual sea-ice distribution and sea-ice drift patterns. The texture, topography and organic content of the surface layer of the seafloor may also regulates the distribution of benthic microorganisms. This surface layer is extremely variable, affected periodically by strong physicochemical gradients due to the movement of tidal waters.
Arctic benthic microalgae typically display striking metabolic adaptations and resilient abilities allowing them to cope up with the extreme ecological variations occurring at the sediment-water interface of intertidal ecosystems. Most benthic microalgae show indeed adaptive diurnal and tidal cycles adjusted with changing light conditions, tide cycles, desiccation, predation and resuspension. Arctic seasonal microphytobenthic variations in biomass typically exhibit a maximum during the spring and summer blooms and periods of low production (oligotrophy) in winter. These maxima are believed to be triggered by enriched coastal nutrient concentration, increasing temperature and increasing day length coincident with coastal pelagic spring blooms. Consecutively, increased grazing pressure slows down microphytobenthic growth considerably. Towards the end of the summer season most microphytobenthic communities undergo changes in composition, coincident with variations in nutrient supplies potentially with the decrease in silicon concentration.
Arctic microphytobenthic primary production
In the Arctic little is known on the productivity of coastal microphytobenthic communities mainly due to the logistic and technical limitations imposed by polar climatic constraints. Furthermore, in benthic tidal habitats productivity measurements may be erroneous and often are difficult to achieve due to large coastal disturbances (sea ice drifting, wind forcing, erosion, desiccation etc. ). The major factors regulating microphytobenthic productivity are typically light availability and the local hydrochemical parameters (salinity, temperature, nutrients, DIN/DIP ratio, etc. ). The very first measurements of arctic marine coastal primary production in the Barents Sea date back to the end of the 1950s and focus mainly on the production of pelagic phytoplankton. More recent estimates were achieved by Dalpadado et al. in 2014, 12 who estimated a mean annual net primary production for the Barents Sea of 59. 0 Tg C year−1. They monitored the evolution of the marine primary productivity over 13 years from 1998 until 2011 in distinct areas of the Arctic Ocean.
Overall, they concluded that the marine net primary production intrinsic to the Barents Sea region is significantly increasing in recent years, from 41. 6 Tg C in 1999 to 80. 9 Tg C in 2011). Moreover, the mean annual net primary production of the Barents Sea was determined to derive from the combinatorial contributions of the Atlantic (53%) coastal (37%) and arctic regions (10%). However, over the whole studied period the specific mean annual net primary production of coastal regions comes out on top. It suggests the significance of the coastal primary production characteristic of Svalbard’s littoral and known as one of the highest across the Arctic Ocean and arctic coastal regions. Arctic benthic microalgae contribute significantly to this coastal productivity. It is believed that in some conditions, when light availability is sufficient and nutrients are locally abundant, benthic primary production may even dominate the net coastal primary production at shallow depths, prevailing over the pelagic production.
Arctic microphytobenthic biodiversity
The arctic microphytobenthos represents a unique and complex diversity of microscopic organisms. This microscopic community of substrate-dwelling phototrophic organisms includes diatoms, cyanobacteria, flagellates, euglenophytes as well as algae. Benthic microphytes selectively occupy the sediment surface of aquatic ecosystems where they are found on all types of substrates, rocks, logs, sand, soft sediments but also as epiphytes on macroalgae and aquatic plants. Distinct taxonomic groups predominate depending on the nature of the physical habitat. Given the large number and dynamic set of environmental variables operating at the arctic coastal interface there are large inter-annual and spatial variations in the composition of coastal arctic communities. The distribution and abundance of the coastal benthic biota is strongly dependant on the local environmental, physicochemical conditions (water temperature, hydrochemistry, salinity, nutrients supply, hydrodynamic, substrate type etc.) and inter-species biological interactions. While some species show a strong affinity for hard substrata, others prefer soft sediment substrates. For instance, the brown alga Chordaria sp. , the green alga Ulotrix sp. and the pennate diatoms Navicula sp. found across the littoral of Svalbard, have been reported to selectively grow on sheltered beaches typically characterized by hard substrates such as large pebbles and stones. Vaucheria sp. studied herein on the contrary, showed a strong affinity for soft sediments.
Despite large coastal disturbance (ice scouring, sea ice) occurring along the Arctic littoral some algae have developed strategies to adapt. In the Arctic, coastal algae typically exhibit fast seasonal growing strategies, making them able to colonise coastal denuded substratum in less than 15 days. Other species tend to grow heterotrophically in winter surviving on nutrient stocks built 13 up during previous favourable growing seasons (e. g. Acrosiphonia sp. and Spongomorpha sp).
