Strong Freshwater Discharge Research Articles

Chlorophyll-a concentration climatology, phenology, and trends in the optically complex waters of the St. Lawrence Estuary and Gulf

The spatiotemporal distribution of phytoplankton biomass drives the marine ecosystem. Chlorophyll-a concentration (Chla) is a biomass index for microalgae in seawater that is commonly used to study phytoplankton by means of satellite remote sensing. The St. Lawrence Estuary and Gulf (SLEG), in Eastern Canada, is a highly dynamic subpolar environment characterized by complex marine optical properties that make it difficult to distinguish Chla from the background signal caused by a strong freshwater discharge. In this study, we implement an inverse model based on a set of in situ Chla processed through a principal component analysis, making it specifically designed for the local marine optical conditions. We used this model to convert a multi-mission remote sensing reflectance dataset to daily Chla between 1998 and 2019 at a 4 km spatial resolution. From the resulting Chla time series, we computed the climatology, phenology and trend over the SLEG. The Chla climatology reveals the presence of relatively high Chla in the Gaspé Current, along the Gulf's North Shore and in areas of strong tidal mixing. Important differences in the phytoplankton phenology between the various subregions are found, with a prevailing shift towards earlier spring blooms of larger intensity. Finally, we found a positive mean Chla increase of 1.1% yr −1 over the SLEG, with strong positive trends in the Magdalen Shallows and west of the Anticosti Island. This description of the surface Chla in the SLEG provides important baseline information for the marine ecosystem. • Our regional algorithm produces realistic chlorophyll-a in the St. Lawrence Estuary and Gulf. • Chlorophyll-a is higher in coastal areas with upwellings or freshwater input. • Spring blooms develop from south to north, with the estuarine portion last. • Between 1998 and 2019, chlorophyll-a increased by 1.1% yr −1 .

Controls on Terrigenous Detritus Deposition and Oceanography Changes in the Central Okhotsk Sea Over the Past 1550 ka

The Okhotsk Sea, which connects the high latitude Asian continent and the North Pacific, plays an important role in modern and past climate changes due to seasonal sea ice coverage and as a precursor of the North Pacific Intermediate Water. The long-term glacial-interglacial changes of sea ice coverage and its impacts on terrigenous transport and surface primary productivity in the Okhotsk Sea remain, however, not well constrained. Base on the paleomagnetic, rock magnetic, micropaleontological (diatom), and geochemical studies of the marine sediment core MD01-2414 (53°11.77′N, 149°34.80′E, water depth: 1,123 m) taken in the central Okhotsk Sea, we reconstruct the terrigenous sediment transport and paleoceanographic variations during the past 1550 thousand years (kyr). Seventeen geomagnetic excursions are identified from the paleomagnetic directional record. Close to the bottom of the core, an excursion was observed, which is proposed to be the Gilsa event ∼1550 thousand years ago (ka). During glacial intervals, our records reveal a wide extension of sea ice coverage and low marine productivity. We observed ice-rafted debris from mountain icebergs composed of coarse and high magnetic terrigenous detritus which were derived from the Kamchatka Peninsula to the central Okhotsk basin. Still during glacial intervals, the initiation (i.e., at ∼900 ka) of the Mid-Pleistocene Transition marks the changes to even lower marine productivity, suggesting that sea-ice coverage became larger during the last 900 ka. During interglacial intervals, the central Okhotsk Sea was either devoid of sea-ice or the ice was at best seasonal; resulting in high marine productivity. The weaker formation of Okhotsk Sea Intermediate Water, lower ventilation, and microbial degradation of organic matter depleted the oxygen concentration in the bottom water and created a reduced environment condition in the sea basin. The freshwater supplied by snow or glacier melting from Siberia and Kamchatka delivered fine grain sediments to the Okhotsk Sea. During the stronger interglacial intervals after the Mid-Brunhes Transition (i.e., Marine Isotope Stages 1, 5e, 9, and 11), strong freshwater discharges from Amur River drainage area are in association with intensified East Asian Summer Monsoon. This process may have enhanced the input of fine-grained terrigenous sediments to the central Okhotsk Sea.

Status of the undisturbed mangroves at Brunei Bay, East Malaysia: a preliminary assessment based on remote sensing and ground-truth observations.

Brunei Bay, which receives freshwater discharge from four major rivers, namely Limbang, Sundar, Weston and Menumbok, hosts a luxuriant mangrove cover in East Malaysia. However, this relatively undisturbed mangrove forest has been less scientifically explored, especially in terms of vegetation structure, ecosystem services and functioning, and land-use/cover changes. In the present study, mangrove areal extent together with species composition and distribution at the four notified estuaries was evaluated through remote sensing (Advanced Land Observation Satellite—ALOS) and ground-truth (Point-Centred Quarter Method—PCQM) observations. As of 2010, the total mangrove cover was found to be ca. 35,183.74 ha, of which Weston and Menumbok occupied more than two-folds (58%), followed by Sundar (27%) and Limbang (15%). The medium resolution ALOS data were efficient for mapping dominant mangrove species such as Nypa fruticans, Rhizophora apiculata, Sonneratia caseolaris, S. alba and Xylocarpus granatum in the vicinity (accuracy: 80%). The PCQM estimates found a higher basal area at Limbang and Menumbok—suggestive of more mature vegetation, compared to Sundar and Weston. Mangrove stand structural complexity (derived from the complexity index) was also high in the order of Limbang > Menumbok > Sundar > Weston and supporting the perspective of less/undisturbed vegetation at two former locations. Both remote sensing and ground-truth observations have complementarily represented the distribution of Sonneratia spp. as pioneer vegetation at shallow river mouths, N. fruticans in the areas of strong freshwater discharge, R. apiculata in the areas of strong neritic incursion and X. granatum at interior/elevated grounds. The results from this study would be able to serve as strong baseline data for future mangrove investigations at Brunei Bay, including for monitoring and management purposes locally at present.

Holocene environmental change at the southern coast of Korea based on organic carbon isotope (δ13C) and C/S ratios

To trace the environmental change that occurred at a coastal area during the Holocene, we performed radiocarbon dating and geochemical analyses using sedimentary cores recovered from the Beolgyo tidal flats on the south coast of Korea. The δ13C values of total organic carbon (TOC), which represent relative inputs of terrestrial (or freshwater) and marine organic sources, revealed a long-term increasing trend from −27.5 to −21.7‰ during the period between 10,000 and 3000 cal BP, suggesting an evolutionary series of fluvial-dominant to marine-dominant environments, coupled with weakened summer monsoon intensity. From 3000 cal BP to the present, the δ13CTOC decreased gradually from −21.7 to −24‰, indicating enhancement of freshwater input during this time. The observed long-term environmental change was divided into six stages based on the averages of the total sulfur content (TS%) and the organic carbon to sulfur ratio (C/S), proxies for paleosalinity. On multi-centennial to millennial timescales, higher C/S ratios and lower TS% were linked to higher δ13CTOC values, indicating strong freshwater input at Stage 2 (∼8300–7400 cal BP), Stage 4 (∼4500–3300 cal BP), and Stage 6 (∼1100 cal BP to present). Strong freshwater discharge events synchronously occurred at 4300 cal BP and 3600 cal BP along the south coast of Korea. Further testing should be carried out in other East Asian coastal areas. This study suggests that sedimentary δ13CTOC values and C/S ratios can be used in Asian coastal regions to trace past freshwater input events and summer monsoon intensity during relatively stable sea level periods (e.g., the middle to late Holocene).

The tephra record from the Sea of Marmara for the last ca. 70 ka and its palaeoceanographic implications

We report the results of tephrochronological studies on marine cores from the eastern and western part of the Sea of Marmara extending back to early MIS-4. Glass shard compositions using electron probe microanalyses (EPMA) and stratigraphic analysis assigned three distinct tephra layers to the eruption of Avellino (Somma-Vesuvius/Italy, 3.9 ka), Cape Riva (Thera/Santorini, 22 ka) and the Campanian Ignimbrite (Phlegrean Fields/Italy, 39.3 ka), respectively. Tephra layers are important chronostratigraphic markers for the refining of age–depth models of cores and thus for the reconstruction of the palaeoceanographic evolution of the Sea of Marmara. Accordingly, a continuous sedimentary record for the last ca. 67 ka in core MD01-2430, located at − 580 m on the Western High, shows only one lacustrine–marine transition at 12.55 ± 0.35 cal ka BP, which indicates that the Sea of Marmara was under lacustrine conditions disconnected from the Mediterranean Sea from early MIS-4 to early MIS-1. High lake levels and oxic bottom water conditions prevailed especially during the MIS-3 stadials. This implies that the sill depth of the Çanakkale Strait (Dardanelles) was probably higher during MIS-3 time than the present sill depth of − 65 m. Alternatively, strong fresh water discharges from the Black Sea during the interstadials might have kept the Sea of Marmara stratified with a relatively thick layer of fresh waters above a weak Mediterranean inflow filling only deeper parts of the basins. Based on the radiocarbon and tephra-integrated age model the marine transgression at 12.55 ± 0.35 cal ka BP was followed by the deposition of the main lower Holocene sapropel between ca. 12.3 and 5.7 cal ka BP, and in the shallow shelf areas (<~100 m depth) such as the Gemlik Gulf, by the formation of the upper Holocene sapropel between ca. 5.4 and 2.7 cal ka BP.

Study on Variability of Residual Current and Salinity Structure According to River Discharge at the Yeoungsan River Estuary, South Korea

The Yeoungsan River Estuary (YRE) made change of estuarine circulation environment at not only control of inflow of freshwater but also decreasing tide speed under construction of estuary bank. The estuarine circulation in the estuary made change rapidly at discharge of the estuary bank. We investigated variation of salinity and residual current in study area during freshwater discharge and non-discharge period. To determine effect of artificial freshwater discharge on the spatial and temporal variability of the residual current and salinity distribution, current and density profile data were analyzed in partially stratified YRE. The current and density data, obtained from two cross-sectional transects, was conducted during freshwater discharge and non-freshwater discharge. We observed flow rate and salt during 13h with 1h interval to investigate changes of marine environment. The residual current structure is complex, such as multi-layer, during non-freshwater discharge because tides and wind effects combined with topography influences. Strong freshwater discharge is influences to vertical mixing at the surface layer,however freshwater is not effected at the bottom layer. These distinction of freshwater effect causes the salinity gradient and strongly stratification. Freshwateris toward to open-sea through surface layer during freshwater discharge period. In other words, the distinction of residual current and the stratification are controlled according to whether artificial freshwater discharge or not.

Depositional environments during the Late Palaeozoic ice age (LPIA) in northern Ethiopia, NE Africa

The Late Palaeozoic sediments in northern Ethiopia record a series of depositional environments during and after the Late Paleozoic ice age (LPIA). These sediments are up to 200m thick and exceptionally heterogeneous in lithofacies composition. A differentiation of numerous types of lithofacies associations forms the basis for the interpretation of a large range of depositional processes. Major glacigenic lithofacies associations include: (1) sheets of diamictite, either overlying glacially eroded basement surfaces or intercalated into the sediment successions, and representing subglacial tillites, (2) thick massive to weakly stratified muddy clast-poor diamictites to lonestone-bearing laminated mudstones originating from a combination of suspension settling of fines and iceberg rainout, (3) lensoidal or thin-bedded diamictites deposited from debris flows, (4) wedges of traction and gravity transported coarse-grained sediments deposited in outwash fans, (5) irregular wedges or sheets of mudstones deformed primarily by extension and incorporating deformed beds or rafts of other lithofacies formed by slumping, and (6) irregular bodies of sandstone, conglomerate and diamictite deformed by glacial pushing.The dominance of laminated or massive clast-bearing mudstones in most successions indicates ice-contact water bodies as the major depositional environment. Into this environment, coarse-grained sediments were transported by various gravity driven transport processes, including dropstone activity of ice-bergs, slumping, cohesive debris flow, hyperconcentrated to concentrated flow, hyperpycnal flow, and by turbidity flow. Close to glacier termini, wedge-shaped bodies of conglomerate, sandstone, diamictite and mudstone were deposited primarily in subaqueous outwash-fans. Soft-sediment deformation of these sediments either records ice push during glacier advance or re-sedimentation by slumping. Apart from an initial glacier advance when thick ice of temperate or polythermal glaciers covered the whole basin, many sections document at least a second major phase of ice advance and retreat, and some sections additional minor advance–retreat cycles.Whether most of the LPIA sediments in northern Ethiopia were deposited in lakes or in fjords is not yet clear. Although univocal evidence of marine conditions is missing, the presence of carbonate-rich beds and the trace fossil assemblage are compatible with a restricted marine environment such as broad palaeofjords affected by strong freshwater discharge during deglaciation. A restricted marine environment for most of the sediments in northern Ethiopia could challenge models of the LPIA sediments in Arabia as primarily glaciolacustrine and glaciofluviatile deposits.

Spring phytoplankton communities shaped by interannual weather variability and dispersal limitation: Mechanisms of climate change effects on key coastal primary producers

Spring bloom composition in the Baltic Sea, a partially ice‐covered brackish coastal waterbody, is shaped by winter‐spring weather conditions affecting the relative dominance of diatoms and a heterogeneous assemblage of cold‐water dinoflagellates, dominated by the chain‐forming Peridiniella catenata and a complex of at least three medium‐sized, single‐celled species: Biecheleria baltica, Gymnodinium corollarium, and Scrippsiella hangoei. During the last decades, the bloom community has dramatically changed in several basins. We analyze here a 30 yr time series of quantitative phytoplankton data, as predicted by hindcast modeled ice thickness and storminess for three distinct Baltic Sea localities, to verify climate‐driven mechanisms affecting the spring bloom composition. Thick (&gt; 30 cm) and long‐lasting ice cover favored diatom‐dominated spring blooms, and mild winters, with storms and thin ice cover (10 to 20 cm), supported blooms of the B. baltica complex. Dispersal limitation plays an important role in the spatial extent of blooms of the B. baltica complex, caused by intricate interplay of local hydrodynamics and the dinoflagellate life cycle. Proportion peaks of key phytoplankton groups have shifted about 10 d earlier in the northwestern Baltic Sea (P. catenata and diatoms) and in the Gulf of Riga (P. catenata). The significant weather effects imply future shifts in spring bloom composition and consequent biogeochemical cycles, driven by the predicted changes in winter storminess and decrease in ice cover extent and duration in climate change models.

IMPACTS OF DEEP WATERWAY PROJECT ON LOCAL CIRCULATIONS AND SALINITY IN THE CHANGJIANG ESTUARY, CHINA

With an aim to improve and maintenance water depth along the navigational channel, the Deep Waterway Project has been conducted in the Changjiang Estuary. The structures of dikes and groins have greatly changed the local circulation with the combined effects of astronomical tide and strong freshwater discharge. A high-resolution fully three-dimension unstructured-grid model (FVCOM) has been applied to study the complicated hydrodynamics with the implementation of unstructured-grid dike-groin module. With the model validation with observation data, the simulation shows the dikes and groins has converted the rotational current into reciprocating flow along the navigational channel between the dikes, and produced the geometrically controlled eddies. The significant southward crossover current was produced along the dike during high tide when the water level is higher than the dike height. The strong saltwater intrusion is also revealed in the observation and model simulation.

Environmental change in the Sea of Okhotsk during the last 1.1 million years

On the basis of two sedimentary records from the central Sea of Okhotsk, we reconstruct the closely coupled glacial/interglacial changes in terrigenous flux, marine productivity, and sea ice coverage over the past 1.1 Myr. The correspondance of our sedimentary records to the China loess grain size record (China loess particle timescale, CHILOPARTS) suggests that environmental changes in both the Sea of Okhotsk area and in SE Asia were closely related via the Siberian atmospheric high‐pressure cell. During full glacial times our records point to a strong Siberian High causing northerly wind directions, the extension of the sea ice cover, and a reduced Amur River discharge. Deglacial maxima of terrigenous flux were succeeded by or synchronous to high‐productivity events. Marine productivity was strengthened during glacial terminations because of an effective nutrient utilization at times of enhanced water column stratification and high nutrient supply from fluvial runoff and sea ice thawing. During interglacials, SE monsoonal winds prevailed, analogous to today's summer situation of a pronounced Mongolian Heat Low and a strong Hawaiian High. Strong freshwater discharge induced by high precipitation rates in the Amur drainage area and a seasonally reduced and mobile sea ice cover favored marine productivity (although being considerably lower than during the terminations) and a lowered flux of ice‐rafted detritus.

The Patos Lagoon hydrodynamics during an El Niño event (1998)

TELEMAC-2D, a two-dimensional depth-averaged finite element flow model, has been chosen for modelling the hydrodynamics of the Patos Lagoon during the 1998 El Niño event. The model is initially calibrated against measurements carried out between 20 and 29/10/98, and good agreement is achieved between measured and predicted longitudinal velocities. Model validation is carried out by comparing measurements and predictions for one reference station in the estuarine area for the period between 23/05 and 06/06/98. Results indicate an excellent agreement between measurements and predictions, and the model is considered calibrated and validated for the Patos Lagoon hydrodynamics.A study of the Patos Lagoon hydrodynamics under the El Niño extreme conditions shows that the local and non-local wind effects associated with the freshwater discharge at the top of the lagoon promote longitudinal and lateral set-up/set-down mechanisms that influence the circulation. Results indicate that velocities in the lagoon and estuary during the extreme conditions observed in the El Niño period are much stronger than the normal periods.The response of the Patos Lagoon entrance to the local and non-local forcing is studied for the first time in terms of the inflow and outflow of water in the system. Results indicate that during the 1998 El Niño event the non-local forcing dominated, promoting exchanges that represent around 7% of the lagoon initial volume in the simulation. Strong freshwater discharges at the top of the lagoon combined with pressure gradients due to changes in elevation generated by the local and non-local wind favours the predominant seawards flow at the mouth, with outflows in the order of 5000m3s−1. The strong outflow observed during the El Niño extreme flow conditions generates huge plumes of freshwater into the adjacent coastal area.