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Mechanisms and causes of green tides with drifting Ulva

What kind of ecological dysfunction can green tides be related to?

Like all macro-algal blooms, green tides with Ulvae are a demonstration of eutrophic conditions in coastal waters.

Eutrophication can be resumed as an “overproduction” of aquatic plants, in connection with excessive inputs of nutrients and leading to a storage of biomass and organic matter in the environment.

However, besides excessive nutrient inputs, two additional conditions are necessary and must be combined in order to trigger eutrophic conditions: good light conditions and availability of an opportunistic species adapted to the local environment (Ulva specie in the case of green tides).

Thus, in general, green tides with Ulva will occur in Brittany, in coastal enclosed sandy areas which are liable to combine excessive input of nutrients (river outlet areas), shallow depths (favorable for algae growth with prevailing appropriate light and temperature conditions) and favorable hydrodynamic conditions able to retain the nutrients and / or the produced algae (i.e slow renewal of the coastal water mass, tidal currents and swells accumulating algae under shallow depths).

Since green tides occur in open seawater masses which experience regular mixing, no dystrophic conditions (critical drop in oxygen levels followed by plant and animal mortality) can be encountered in opposition to what can be observed with seaweed blooms in closed lagoons. Only stagnant and poorly drained beach strandings located on the upper fringe of foreshores can create, through the decomposition of the biomass and on a smaller scale, these dystrophic conditions.

What is the effect mechanism of nutrients?

The occurrence and then the level of significance of green tides, on a specific site, depend more precisely on the remanence and amount of available nutrients during an appropriate season for algae growth.

Green algae, along with other marine primary producers, are in general limited in their growth during the summer season (from May onwards), due to the natural and significant decline of available nutrients (depletion of stock nutrients by phytoplankton, reduction of their contributions by the rivers). Eutrophication of a site is linked to a seasonal delay and to a weakening of this natural limiting factor. These two factors, which are linked to the overall increase of continental nutrient inputs, enable the algae to continue their growth phase under increasingly favorable seasonal conditions such as light and temperature, and to accumulate high amounts of biomass in sandy bays, where such algae are initially absent.

What are the respective roles of nitrogen and phosphorus?

Both nitrogen and phosphorous are necessary for the growth of Ulvae. By measuring the nitrogen and phosphorus levels in the algae itself, phosphorus is found to be almost constantly above levels required for the growth of algae, while nitrogen remains a limiting growth factor during summer on most measured sites.

Please note !

In spring, nutrient availability in the environment decreases rapidly as the need for algae growth increases, resulting in seasonal declines of internal N and P concentrations in Ulvae. It is only during summer that nitrogen levels fall below the threshold for algae growth.

Therefore, nitrogen appears as a ‘minimum’ or ‘limiting’ factor for algal production, this indicates that by adding more of this nutrient to the growth media, algae proliferation will increase and on the other hand, removing nitrogen will decrease proliferation in direct proportion. However, on many measured sites, nitrogen availability has increased to levels above algal growth requirements. Phosphorus, on the other hand, seems to play only occasionally and / or temporarily a direct role in limiting growth and this only on certain sites.

In addition, nitrogen and phosphorus have different behaviors in coastal environments: when nitrogen is introduced in the open sea it is instantly diluted, while phosphorus precipitates and tends to accumulate in coastal sediments. This phosphorus, hence trapped in high amounts, would however (unlike the situation in fresh water) at least be partly and easily released by the sediment, which would explain its particular availability for Ulva. At the same time, this large stock of phosphorus accumulated in the coastal sediments explains why this nutrient is not a control factor for algae growth, in comparison to nitrogen who has no storage system in coastal environments and is hence limiting for algae growth.

What could be done to reduce the growth of green algae?

The only long-term solution is to reduce the nutrient supply in coastal waters. With regards to the reasons mentioned previously, nitrogen is the main operating lever to focus on in order to act quickly against green tides with Ulva on the coasts of Brittany. This nutrient is found under the nitrate form, mainly from agricultural origin. Therefore, control efforts must be concentrated within agricultural activities in order to act on the phenomenon of green tides and, more generally, to restore the quality of coastal water masses with respect to eutrophication risk (seaweed and phytoplankton blooms). It should be noted, however, that many efforts have been made by the agricultural sector in recent years.

Green tide with Ulva susceptibility factors

Seawater transparency :

In addition to nutrients, light is the second most important factor in eutrophication. The specific development of green tides on the coasts of Brittany is most likely fostered by natural low water turbidity (rocky coast, no large rivers). Indeed, sufficient light for a significant growth of Ulva seems possible in some sites, reaching up to 10-15 m depth. On the other hand, the absence of green algae in the Mont St Michel Bay is explained by the prevailing higher water turbidity.

The geomorphological and hydrodynamic factors of the coastline

The importance, or even the existence, of green tides on a specific site also depends on the geomorphological and hydrodynamic capacity of the coastal area:

  • to reduce the dilution of nutrients which are responsible for the growth of algae.

Indeed, despite the great tidal ranges, renewal of coastal water masses enriched with nutrients is more or less limited in many landlocked and shallow areas of the Brittany coastline.

  • to store the produced algae in shallow depths and to promote their stranding.

This storage of biomass is to be considered, at first, during the summer period, where the highest amounts are reached and the most nuisance is involved. The total stock is divided between the foreshore (+ floating curtain of bottom of beach) and shallow waters which are not accessible for direct observation. Estimates of total biomass on different sites tend to show that the amount of biomass pushed towards and on the beaches, is just as high as the tidal range is strong in the considered coastal area.

The storage of biomass is also to be considered in winter as it stands for residual stocks of the previous year and enables an anticipated bloom at the end of the winter season, just under the simple control of light conditions which are improved.

As a result, a maximum amount of algae will enter the optimal period for algal growth (temperature + light + nutrients) and accumulate in large quantities before undergoing nitrogen limitation in summer. The monitoring of green tides over several years has shown that the peak of green tides in spring (June) depend very little on quantifiable nitrogen fluxes before and during this period. On the other hand, these peak events could be correlated to available Ulva stocks on the different sites at the end of the previous season, as well as to the winter dispersion and growth conditions of the algae, overall allowing more or less important biomasses to enter the sites at the end of winter, and thus according to the years.

This winter stock is variable from one site to another according to the local hydrodynamic dispersion conditions of the algae but can be considered as an inertia factor for the interannual variation of the algal summer stocks of a site, in comparison to variation fluxes, as well as a potential resistance factor of sites towards preventive measures.

Finally, when considering an affected bay, it is necessary to take into account the possibility of importing nutrients and especially seaweed biomass (which seems, in fact, the most frequent) by lateral transport, coming from neighboring sites. This transport enables an increase in the maximum stock of algae or again can provide sites, which initially do not have green algae blooms, with algae stocks at the beginning of the season. This possible interdependence between neighboring sites is an important parameter to be evaluated (by hydrodynamic studies). Indeed, this will help to group territorial control efforts which still operate independently on neighboring river-basins, and to better target harvesting efforts on such bay or part of the bay which is liable to be the source of contamination for other areas.

The potential aggravating role of natural climatic, geological and land-use factors on watersheds

The geological nature of the subsoil, which controls the importance of minimum flows in rivers and how early it can occur, and the type of land use in the river-basin, in particular the development and farming practices which promote direct pathways for water and reduces possibilities for natural denitrification, can both worsen the seasonal patterns of nitrogen transfer to a site exposed to green tides and during a sensitive period.

This explains why granitic river-basins experience minimum flow rates which are late in the season and with nutrient inputs (flux) that may be sufficient for Ulva growth over the summer period. The climatic conditions which prevail in Brittany (mild winter temperatures, strong rainfall) are generally favorable for mineralization and nitrogen transfer the ground water, but this in variable conditions from one basin to another.

Summary diagram describing conditions for the development of typical green tides in sandy bays in Brittany