Understanding how physical-biological coupling influences harmful algal blooms, low oxygen and fish kills in the Sea of Oman and the Western Arabian Sea

Gradual decadal changes have taken place in the Western Arabian Sea over the last 50 years. These changes have affected wind speeds, atmospheric and sea surface temperature, thermohaline stratification, shoaling of the oxycline, and dust/iron inputs. A decrease in nitrate supply of the photic layer have caused an increase in annual frequency of harmful algal blooms and fish kills. Along with that, a decrease in diatom biomass and a shift from red Noctiluca to green Noctiluca during the northeast monsoon was observed during the last two decades, Even though these are the same species they have very different nutritional modes. The red one is a heterotroph with a preference for grazing diatoms, while the green one has a symbiont and thus it is a mixotroph. Recent results suggest that this shift may be caused by the shoaling oxycline since the green one grows better under low oxygen because the symbiont produces oxygen for its host. The western Arabian Sea is temporally and spatially complex. With the recent advances in remote sensing of the ocean, a further understanding of these temporal and spatial changes can be gained through analyzing frequent images with opportunistic ground-truthing.

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Overview of Decadal Ecosystem Changes in the Western Arabian Sea and the Occurrence of Algal Blooms

Journal of Agricultural and Marine Sciences [JAMS] ◽

10.24200/jams.vol23iss1pp11-23 ◽

2019 ◽

Vol 23 (1) ◽

pp. 11

Author(s):

Paul Harrison ◽

Sergey Piontkovski ◽

Khalid Al-Hashmi

Keyword(s):

Harmful Algal Blooms ◽

Algal Blooms ◽

Arabian Sea ◽

Northeast Monsoon ◽

Low Oxygen ◽

Wind Speeds ◽

Spatial Changes ◽

Ground Truthing ◽

Ecosystem Changes ◽

Thermohaline Stratification

Gradual decadal changes have taken place in the Western Arabian Sea over the last 50 years. These changes have affected wind speeds, atmospheric and sea surface temperature, thermohaline stratification, shoaling of the oxycline, and dust/iron inputs. A decrease in nitrate supply of the photic layer have caused an increase in annual frequency of harmful algal blooms and fish kills. Along with that, a decrease in diatom biomass and a shift from red Noctiluca to green Noctiluca during the northeast monsoon was observed during the last two decades, Even though these are the same species they have very different nutritional modes. The red one is a heterotroph with a preference for grazing diatoms, while the green one has a symbiont and thus it is a mixotroph. Recent results suggest that this shift may be caused by the shoaling oxycline since the green one grows better under low oxygen because the symbiont produces oxygen for its host. The western Arabian Sea is temporally and spatially complex. With the recent advances in remote sensing of the ocean, a further understanding of these temporal and spatial changes can be gained through analyzing frequent images with opportunistic ground-truthing.

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Responses of food web to hypolimnetic aeration in Lake Vesijärvi

Hydrobiologia ◽

10.1007/s10750-020-04319-6 ◽

2020 ◽

Vol 847 (21) ◽

pp. 4503-4523 ◽

Cited By ~ 3

Author(s):

Jukka Ruuhijärvi ◽

Tommi Malinen ◽

Kirsi Kuoppamäki ◽

Pasi Ala-Opas ◽

Mika Vinni

Keyword(s):

Body Size ◽

Food Web ◽

Harmful Algal Blooms ◽

Algal Blooms ◽

Perca Fluviatilis ◽

Fish Predation ◽

Filamentous Cyanobacteria ◽

Low Oxygen ◽

High Quality ◽

Oxygen Conditions

AbstractWe studied the responses of a food web, especially fish and zooplankton, to summertime aeration, pumping of oxygen-rich epilimnetic water to the hypolimnion in Lake Vesijärvi, southern Finland. The aim of hypolimnetic aeration was to reduce internal loading of phosphorus from sediment. The population of smelt (Osmerus eperlanus L.), the main planktivore of the pelagial area, collapsed during the two 1st years of aeration due to increased temperature and low oxygen concentrations in the hypolimnion. The population recovered after the 4th year of hypolimnetic aeration, when oxygen conditions were improved. Despite elevated hypolimnetic temperature, smelt reached exceptionally high abundance, which led to a significant reduction in cladoceran body size. The density of perch (Perca fluviatilis L.) increased at first, but then decreased when the proportion of smelt and cyprinids increased. Biomasses of Daphnia decreased probably as a result of the disappearance of dark, low-oxygen deep-water refuge against fish predation and low availability of nutritionally high-quality algae. Occasionally filamentous cyanobacteria, such as turbulence tolerant Planktothrix agardhii (Gomont), were abundant, suggesting deteriorated food resources for zooplankton. The responses of food web were controversial with respect to the aim of the management, which was to prevent the occurrence of harmful algal blooms.

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Challenges in modeling spatiotemporally varying phytoplankton blooms in the Northwestern Arabian Sea and Gulf of Oman

Biogeosciences ◽

10.5194/bg-13-1049-2016 ◽

2016 ◽

Vol 13 (4) ◽

pp. 1049-1069 ◽

Cited By ~ 5

Author(s):

S. Sedigh Marvasti ◽

A. Gnanadesikan ◽

A. A. Bidokhti ◽

J. P. Dunne ◽

S. Ghader

Keyword(s):

Interannual Variability ◽

Annual Cycle ◽

Harmful Algal Blooms ◽

Algal Blooms ◽

Arabian Sea ◽

Regional Scale ◽

Phytoplankton Productivity ◽

Gulf Of Oman ◽

The North ◽

The Persian Gulf

Abstract. Recent years have shown an increase in harmful algal blooms in the Northwest Arabian Sea and Gulf of Oman, raising the question of whether climate change will accelerate this trend. This has led us to examine whether the Earth System Models used to simulate phytoplankton productivity accurately capture bloom dynamics in this region – both in terms of the annual cycle and interannual variability. Satellite data (SeaWIFS ocean color) show two climatological blooms in this region, a wintertime bloom peaking in February and a summertime bloom peaking in September. On a regional scale, interannual variability of the wintertime bloom is dominated by cyclonic eddies which vary in location from one year to another. Two coarse (1°) models with the relatively complex biogeochemistry (TOPAZ) capture the annual cycle but neither eddies nor the interannual variability. An eddy-resolving model (GFDL CM2.6) with a simpler biogeochemistry (miniBLING) displays larger interannual variability, but overestimates the wintertime bloom and captures eddy-bloom coupling in the south but not in the north. The models fail to capture both the magnitude of the wintertime bloom and its modulation by eddies in part because of their failure to capture the observed sharp thermocline and/or nutricline in this region. When CM2.6 is able to capture such features in the Southern part of the basin, eddies modulate diffusive nutrient supply to the surface (a mechanism not previously emphasized in the literature). For the model to simulate the observed wintertime blooms within cyclones, it will be necessary to represent this relatively unusual nutrient structure as well as the cyclonic eddies. This is a challenge in the Northern Arabian Sea as it requires capturing the details of the outflow from the Persian Gulf – something that is poorly done in global models.

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Effects of Harmful Blooms of Large-Sized and Colonial Cyanobacteria on Aquatic Food Webs

Water ◽

10.3390/w12061587 ◽

2020 ◽

Vol 12 (6) ◽

pp. 1587 ◽

Cited By ~ 1

Author(s):

Maria Moustaka-Gouni ◽

Ulrich Sommer

Keyword(s):

Harmful Algal Blooms ◽

Algal Blooms ◽

Nutritional Value ◽

Cyanobacterial Blooms ◽

Crustacean Zooplankton ◽

Fish Production ◽

Fish Kills ◽

Fish Yield ◽

Grazing Resistance ◽

Aquatic Food

Cyanobacterial blooms are the most important and best studied type of harmful algal blooms in fresh waters and brackish coastal seas. We here review how and to which extent they resist grazing by zooplankton, how zooplankton responds to cyanobacterial blooms and how these effects are further transmitted to fish. Size, toxicity and poor nutritional value are widespread mechanisms of grazing defense by cyanobacteria. In some cases, defenses are inducible, in some they are obligate. However, to some extent zooplankton overcome grazing resistance, partly after evolutionary adaptation. Cyanotoxins are also harmful to fish and may cause fish kills. However, some fish species feed on Cyanobacteria, are able to reduce their abundance, and grow on a cyanobacterial diet. While reduced edibility for crustacean zooplankton tends to elongate the food chain from primary producers to fish, direct feeding by fish tends to shorten it. The few available comparative studies relating fish yield to nutrients or phytoplankton provide no indication that cyanobacteria should reduce the ratio fish production: primary production.

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What Happened in 1950?

Dead Zones ◽

10.1093/oso/9780197520376.003.0005 ◽

2021 ◽

pp. 52-71

Author(s):

David L. Kirchman

Keyword(s):

Gulf Of Mexico ◽

Mississippi River ◽

Organic Material ◽

Harmful Algal Blooms ◽

Algal Blooms ◽

Mark Twain ◽

Dead Zone ◽

Algal Growth ◽

Fish Kills ◽

Dead Zones

This chapter discusses what happened around 1950 that led to the expansion of dead zones. For the Gulf of Mexico, there are many reasons to think the flow of the Mississippi River has changed since the days of Mark Twain, considering the construction of so many levees, dikes, floodways, spillways, weirs, and revetments. Rain-absorbing grasslands and forests have been replaced by asphalt, roof shingles, and other hydrophobic material that hasten rainwater to the Gulf. But the flow of the Mississippi has not changed enough to explain why the Gulf dead zone grew around 1950. As the chapter discusses, what did change was nutrients. It shows that concentrations doubled in the Mississippi River from the 1930s to the 1990s, which stimulated algal growth and production of organic material that eventually led to depletion of dissolved oxygen. In addition to creating dead zones, the increase in nutrients has stimulated harmful algal blooms, leading to fish kills and beach closings.

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