Why are upwellings important to plankton

Thus, the phytoplankton community was relatively stable in the area in July and the high biomass community observed on 8 August in the northern part of the study transect matches with the July community. MDS plot for all sampled stations based on Bray—Curtis similarity distances of phytoplankton composition.

Filled triangles mark the stations sampled in the upwelling areas. The stations sampled during the major upwelling event in the southern part of the study transect were grouped in area 2 1c—7c and 1d—11d. Station TH1 was not placed at the same point in different weeks as the bigger share of Chrysochromulina spp. Area 3 includes the stations from the central part of the study area 9c—21c sampled on 8 August where the phytoplankton community clearly differed from both the July community and the upwelling water community.

Dissimilar communities to all those described above were observed on 15—16 August in the areas outside the upwelling waters, whereas the community in the upwelling front 13d was closer to the upwelling community than to the communities outside the upwelling area. The phytoplankton community at station TH13 13e on 22 August matches the same group, while the community in a colder water patch at station TH11 11e was slightly different.

The communities at the rest of stations on 22 August were close to the community observed at stations TH21—TH27 21d—27d a week earlier. Area 4 includes the stations sampled in the upwelling area on 29 August. Those communities differed from the ones observed during the previous upwelling event in the same area. The communities in the northern half of the study transect, which stayed outside the upwelling influence on 29 August, diverged from the previously observed communities as well.

Patti et al. Wind-driven transport and mixing play three key roles in shelf productivity: nutrient supply to the euphotic zone, light control through vertical mixing of primary producers and offshore export of near surface plankton.

However, while the first process yields a strong positive biological response to upwelling winds, the other two influences lead to negative productivity responses to increased wind Largier et al.

During the coastal upwelling events in the Baltic Sea, the distribution of phytoplankton biomass and production have been found to be strongly influenced by the combined effects of physico-chemical processes and to be related to the strength of wind and hence upwelling events Kononen and Niemi, ; Vahtera et al. In the Gulf of Finland, there is a strong upwelling—downwelling coupling because of the elongated shape and relatively short distance 48— km between northern and southern coasts.

In general, a strong biomass decrease in the area affected by the major upwelling event and an increase of biomass near the opposite coast was observed, and there was strong evidence of negative and positive effects of this event on different phytoplankton species. In plankton community studies very often phytoplankton are divided into several size classes. At low nutrient levels, phytoplankton species succession usually proceeds towards small and motile species with high surface to volume ratios, which give an advantage in nutrient uptake e.

In ocean upwelling systems, larger and especially diatom species are considered to dominate the phytoplankton community e. Lassiter et al. In contrast to the general understanding and expectations, in our observations the smaller phytoplankton size fraction showed a faster response to the upwelled nutrients compared with the larger size fraction.

It was shown by Kuvaldina et al. Kuvaldina et al. Stolte et al. Koike et al. The median and mean values were 1. Lignell et al. Thus, we suggest that the phytoplankton growth observed in the upwelling area, initially mostly in the smaller size fraction Pyramimonas spp. As shown by the previous studies in the Gulf of Finland Kanoshina et al. During our study, Eutreptiella spp. There was no biomass increase observed in connection to the major upwelling event along the southern coast in August.

Pyramimonas spp. Chrysochromulina spp. Thus, these species have a high requirement for nutrients, and hence might have a competitive advantage during upwelling events which explain the increase in their biomass in the major upwelling area in our study.

As shown by studies of Hajdu et al. Hajdu et al. New input of limiting nutrients during the upwelling event at the beginning of August together with the stabilization of upwelling and rise of the UML temperature leading to development of stratification might have caused the Chrysochromulina spp. Also phosphorus deficiency in the UML gives a competitive advantage through toxin production Estep and MacIntyre, and hence helps to explain the high abundances at periods with low phosphate concentrations e.

Among larger size fraction, the biomass of the photosynthetic ciliate M. The biomass increase of latter species during the upwelling event was first connected with the vertical transport of seed population from lower layers followed by growth in the nitrate-rich upwelled waters in the euphotic layer. The former conclusion is supported by sampling on 11 July at stations TH1—TH3 where in the lower part of thermocline High biomass values and even blooms of M.

The species has been shown to have a high demand for inorganic nitrogen, but also one to four times higher uptake rate for nitrate than e. The latter helps also explain the rapid exhaustion of inorganic nitrogen compounds in M. The competitive advantage during nutrient limitation is retained probably by changing the trophic status from phototrophy to phagotrophy e. Yih et al. Lindholm, ; Villarino et al. We noticed that the highest H. Kononen et al. Since the Chl a fluorescence in the UML at these stations was remarkably high data not shown here , we suggest that H.

This suggestion is supported by the CTD data from 19 July showing that the isopycnal, which surfaced in the upwelling front station TH25 , was located at 12 m depth at a distance of 5 km towards the open gulf station TH Vahtera et al. We observed similarly relatively high biomass of Aphanizomenon sp. However, the biomass of Aphanizomenon sp. It could be explained by the fact that the upwelled water during the major event near the southern coast had its origin from much deeper layers than that during the earlier upwelling in the northern part.

It was estimated by Lips et al. Lips et al. There was a clear increase in phytoplankton Chl a and biomass after the relaxation of the major upwelling sampled on 22 August Fig. However, the highest total phytoplankton biomass was observed on 8 August in the northern part of the study transect. In contrast, the phosphate concentrations exceeding the lower detection range were observed in the UML in the northern part of the study transect on 25 July and 8 August. Most marine fish and invertebrates produce microscopic larvae which, depending on the species, may drift in the water for weeks or months as they develop.

For adult marine creatures that live in shallow waters nearshore, upwelling that moves surface water offshore can potentially move drifting larvae long distances away from their natural habitat, thus reducing chances for survival. In some ways, upwelling can be a mixed blessing to coastal ecosystems. It can infuse coastal waters with critical nutrients that fuel dramatic productivity, but it can also rob coastal ecosystems of offspring required to replenish coastal populations.

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