Cross-shelf advection as a mechanism of regional climate change influence on plankton community in the coastal waters

Yury Zuenko and Victoria Nadtochy

Pacific Fisheries Research Center (TINRO), Vladivostok, Russia

Goal: to understand the mechanism of climate change influence on plankton community of coastal zone in case of Peter the Great Bay, Japan/East Sea

The Suyfen is one of the largest rivers of the Japan/East Sea basin with annual discharge about 2.2 km3 of fresh water

Dynamics of annual precipitation in Vladivostok: positive tendency is observed both for winter and summer

Суша ветер Море Суша ветер Море весна-лето: осень-зима: даунвеллинг апвеллинг

Scheme of on-shore and off-shore monsoon winds influence on cross-shelf water circulation and topography of pycnocline in the coastal zone of Primorye

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Vladivostok

The ecosystem of Peter the Great Bay is influenced by factors of both terrestrial and marine environments, as fresh water discharge and cross-shelf advection, and both these factors are subjected to changes of climate scale.

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Schemes of winter and summer monsoon formation

Climate change background Large-scale climate changes in the Japan/East Sea area are conditioned by changes in Asia-Pacific monsoon system where trends to weakening of monsoon winds, in particular in winter, prevail in modern times. Inter-decadal fluctuations are observed on this background.

Dynamics of winter monsoon intensity (Siberian High Index). Inter-decadal fluctuations are observed on the background of negative trend

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Dynamics of summer monsoon intensity in its first phase (Okhotsk Index) and second phase (North Pacific Index). Inter-decadal fluctuations are observed on the background of negative trends, but the trend is statistically insignificant for the 2nd phase

Neritic copepods

Deep-water copepods

Sagitta

Cladocera

Others

Change of zooplankton community Large-scale climate changes in the Japan/East Sea area are conditioned by changes in Asia-Pacific monsoon system where trends to weakening of monsoon winds, in particular in winter, prevail in modern times. Inter-decadal fluctuations are observed on this background.

1980s 1990s 2000s Total biomass: 1093 mg/m3 2311 mg/m3 1555 mg/m3 Deep-water migrants: 29% 65% 25% Species structure of early-summer zooplankton community in the coastal zone of Peter the Great Bay, by decades

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Data description

Several research programs were realized in the coastal zone of Peter the Great Bay, mostly in the Amur Bay, since 1980. The observations were the most frequent in the middle of 1980s, in the late 1990s, and in the 2007-2014, so now the series of irregular but frequent data exceeds 3 decades that means that they are enough long for evaluation of climate changes

Some examples of the surveys in the coastal zone of Peter the Great Bay conducted with different research programs after 1980

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Data processing

To avoid influence of spatial inhomogeneity, variations of zooplankton and oceanographic factors were averaged within small (6 x 6 miles) areas in the coastal zone and considered separately

Scheme of averaging within small areas of the coastal zone (a case of the Amur Bay).The data quantity in the yellow-colored areas is insufficient for he study

Data processing

To exclude seasonal variations, seasonal decomposition was executed for all oceanographic data, and only one biological season was considered for zooplankton (early summer = June)

Examples of mean annual changes of water temperature, salinity, dissolved oxygen content, and phosphorus concentration at the sea surface (top panel) and at the sea bottom (20 m depth - bottom panel) in different small areas in the coastal zone of Peter the Great Bay

Sea surface temperature

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0 30.6 61.2 91.8 122.4 153 183.6 214.2 244.8 275.4 306 336.6

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Sea surface salinity

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Dissolved oxygen contentat the sea surface

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Temperature at 20 m

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0 30.6 61.2 91.8 122.4 153 183.6 214.2 244.8 275.4 306 336.6I II III IV V VI VII VIII IX X XI XII

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Salinity at 20 m

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Dissolved oxygen content at 20 m

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Zooplankton changes

Abundance and biomass of zooplankton species, as well as total abundance and biomass of zooplankton, have both year-to-year changes and certain tendencies in the climatic scale. The most of species have positive trend. The only exclusion among the mass groups of species are arrowworms, mainly Sagitta elegans, which become less abundant. They are allochtonous, so they spend winter and reproduce in the deep-water sea. Other mass species, which became more abundant, are neritic ones which reproduce in the coastal zone.

Biomass difference between 1980-1996 and 1998-2012 for mass species in the coastal zone of Peter the Great Bay

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Zooplankton species in the southern Amur Bay in June

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Ps.newmani Sagitta

Year-to-year changes of the arrowwarms biomass in compare with the total biomass of zooplankton (upper panel) and the biomass of small-sized copepod Pseudocalanus newmani (bottom panel). The arrowwarms and other species have opposite trends of the biomass

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total zooplankton Sagitta

Zooplankton changes vs summer monsoon changes

The negative tendency of arrowworms abundance is observed in the coastal and shelf zones only but is absent in the deep area of the Japan Sea. So, the state of the arrowworms population is stable, and recent decreasing of their abundance in the coastal zone is caused by weakening of their cross-shelf transport. These changes in zooplankton community coincides with weakening of summer monsoon that obviously drives the cross-shelf transport (r = 0.49).

So, decreasing of the migrants abundance could be reasoned by summer monsoon weakening. But what is the reason of local species blooming?

Year-to-year changes of the deep-water migrants portion in zooplankton of the coastal waters and the Okhotsk Index of summer monsoon intensity in June

Year-to-year changes and trends of Sagitta biomass in the coastal zone of Peter the Great Bay and adjacent deep-water area of the Japan Sea in June

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Sagitta in the deep-sea Sagitta on the shelf

Суша ветер Море Суша ветер Море весна-лето: осень-зима: даунвеллинг апвеллинг

Year-to-year changes of mean summer SST anomaly in different areas of the coastal zone – positive trend is observed everywhere

Year-to-year changes of mean summer salinity anomaly at the sea surface in different areas of the coastal zone – there are no statistically significant trends

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Estuary West East

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приэстуарная зона западные участки восточные участки

Temperature and salinity

Temperature and salinity have similar year-to-year changes in different areas of the coastal zone in Peter the Great Bay and in different months within the seasons. The sea surface temperature has a tendency to increasing, and salinity doesn’t have a statistically significant trend.

Temperature and salinity

Temperature at the sea bottom has no trend in the early summer and ha negative trend in the late summer. Salinity at the sea bottom has no any statistically significant trend.

Year-to-year changes of temperature anomalies at the 20 m depth in the coastal zone (at the sea bottom)

Year-to-year changes of salinity anomalies at the 20 m depth in the coastal zone (at the sea bottom)

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Precipitation V+VI+VII

Nutrients In spite of the river discharge increasing into the coastal zone of Peter the Great Bay (mainly from the Suyfen River), negative tendency of the nutrients concentration is observed for the coastal zone in summer. This tendency could be caused by strengthening of density stratification because of warming and freshening of the surface layer in the coastal zone. So, potential productivity of the coastal waters is decreased.

Phosphate at the sea surface in the eastern Amur Bay

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Phosphate at the sea surface in the western Amur Bay

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Difference of the mean concentrations of inorganic phosphorus at the sea surface in two areas of the coastal zone between 1980s and 2000s. The negative difference means a decreasing of concentration

Year-to-year changes and trend of summer precipitations in the Suyfen River valley

Dissolved oxygen at the sea bottom

Hypoxia at the sea bottom in some areas of the coastal zone in Peter the Great Bay became easier in the 2000s, in compare with the 1980-1990s. It is another evidence of lowering productivity for the coastal waters.

Difference of the mean dissolved oxygen content at the sea bottom in two areas of the coastal zone between 1980-1990s and 2000s. The content has increased for the period of hypoxia (late summer)

Dissolved oxygen at 20 m depth in the western Amur Bay

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Dissolved oxygen at 20 m depth in the eastern Amur Bay

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In conditions of recent weakening of high-pressure atmospheric centres, as the Okhotsk High, the summer monsoon is weakening that means for the coastal zone a lowering of atmospheric pressure, increasing of frontal rains, decreasing of cloudiness, weakening of stable monsoon winds, and as the result a warming of the sea surface. Secondary effect of the warming is a strengthening of density stratification and weakening of turbulent mixing in the period of the highest SST, that’s why deep layers are cooling.

Conclusion: mechanisms of climate changes downscaling to the ecosystem of coastal waters in Peter the Great Bay – 1) abiotic components

Hawaiian High

Asian Low

FEL

FEL = Far Eastern Low OH = Okhotsk Sea High

OH

Scheme of summer monsoon formation

Summer precipitations in the Suyfen valley

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Precipitation V+VI+VII

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Dynamics of summer monsoon intensity in its first phase (Okhotsk Index)

y = 0.03x - 54.40

y = 0.04x - 74.75y = 0.07x - 148.52

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Year-to-year changes of mean summer SST anomaly in different areas of the coastal zone

warmer stronger weaker sea surface stratification monsoon weaker cross- shelf exchange

Another effect of the stratification strengthening is a weakening of nutrients upward influx to the photic layer that potentially prevents the primary producers growth. As the result of supposed decreasing of phytoplankton concentration, summer hypoxia at the sea bottom became weaker

Hawaiian High

Asian Low

FEL

FEL = Far Eastern Low OH = Okhotsk Sea High

OH

Phosphate at the sea surface in the western Amur Bay

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Dissolved oxygen at 20 m depth in the western Amur Bay

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Conclusion: mechanisms of climate changes downscaling to the ecosystem of coastal waters in Peter the Great Bay – 1) bioproductivity

Scheme of summer monsoon formation

Difference of the mean concentrations of inorganic phosphorus at the sea surface in two areas of the coastal zone between 1980s and 2000s. The negative difference means a decreasing of concentration

stronger lower lower stratification nutrients production

Decreasing of phytoplankton abundance in the coastal zone in summer obviously is not danger for the consumers because of surplus of its production – mass local species have enough food to maintain their populations.

Abundance of some allochtonous species, as the deep-water copepods N.plumchrus and O.similis, is decreasing in the coastal zone because of weakening of their cross-shelf wind-driven transport, but it is not essentially important for total biomass of zooplankton. Abundance of the most numerous allo-chtonous species, as the predatory Sagitta elegance, becomes lower, too, that has a positive effect on mass copepods in the coastal zone because of lower predatory.

Conclusion: mechanisms of climate changes downscaling to the ecosystem of coastal waters in Peter the Great Bay – 3) zooplankton

Sagitta elegans

Oithona similis

Neocalanus plumchrus

Pseudocalanus newmani

Acartia clausi

Cladocera

summer monsoon wind

Scheme of deep-water migrants transport to the coastal waters by cross-shelf circulation driven by summer monsoon

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Zooplankton species in the southern Amur Bay in June

Biomass difference between 1980-1996 and 1998-2012 for mass species in Peter the Great Bay

lower production no visible effect higher weaker cross- lower zooplankton shelf exchange predatory abundance

Thus, climate changes cause lowering of productivity in coastal ecosystem of Peter the Great Bay, but zooplankton community of this area is top-controlled, and processes related to cross-shelf advection provide heightening of zooplankton abundance