Adjacent Coastal Region Research Articles

Silicon isotopes reveal the impact of fjordic processes on the transport of reactive silicon from glaciers to coastal regions

Accelerated mass loss from the Greenland Ice Sheet leads to retreating glaciers and enhanced freshwater runoff to adjacent coastal regions, potentially providing additional essential nutrients, such as silicon, to downstream primary producers. However, the role of fjordic sediments in modulating the supply of silicon from glacial environments to marine ecosystems remains poorly constrained, particularly for the quantification of silicon fluxes from the sediments into overlying waters in high-latitude fjordic systems. In this study, we use the concentration and stable isotopic composition of dissolved silicon in pore waters and core-top waters, and amorphous silica phases (such as glacially-derived amorphous silica) in sediments and suspended particulate matter, collected from two fjords in the southwest Greenland margin to address this knowledge gap. We combine downcore observations with core incubations and isotope mass balance approaches to assess the benthic flux of dissolved silicon and deconvolve potential contributors to this flux during early diagenesis. Our results suggest that molecular diffusion only accounts for a portion of benthic dissolved silicon transport. Relative to surrounding continental shelves and highly-productive open ocean waters, the estimated benthic dissolved silicon flux at our sites is smaller in magnitude, supporting the role of fjords as a ‘trap’ for reactive silicon in high-latitude systems.

Possible influence of the large-scale environment on extreme waves over the Yellow Sea in boreal spring

The Yellow Sea (YS) is exposed to various weather systems, such as typhoons, monsoon activities, and extratropical cyclones, which can pose a major threat to the adjacent coastal regions through the development of energetic oceanic surface waves. Unusually severe surface wave events in the YS occur with considerable frequency during the boreal spring (March-April-May), but have received less attention compared to winter and summer. This study focuses on the characteristics of spring extreme wave height (EWH) events in the YS during 1979-2022, based on observational and long-term reanalysis datasets. Our analysis shows that extreme waves, i.e. daily maximum heights in the top 5% of all spring days, take about 12 h to build up to the peaks, while they decay more slowly after the peaks. During the extreme events, the Siberian High is found to extend anomalously eastward compared to spring climatology. Such an anomalous extension contributes to the increase of the sea level pressure gradient and the intensification of the surface wind speed in the YS. Meanwhile, in the range of 6 ∼ 24 h following the peaks of the EWH events in the YS, swells induced by strong northerly winds begin to have an impact in the YS. These swells contribute to maintaining higher wave energy levels in the YS for longer after the atmospheric source has been removed. We further explore the large-scale environmental conditions that could provide the predictability of extreme waves in the basin developed by these findings. This study presents implications for assessing the risks associated with extreme waves in coastal regions and for improving coastal management strategies in the YS.

Operational wind forecast over the offshore Campos basin and adjacent coastal region in Brazil: support for the oil and gas industry toward renewable energy transition

Operational wind forecast over the offshore Campos basin and adjacent coastal region in Brazil: support for the oil and gas industry toward renewable energy transition

Distinct Radiocarbon Ages Reveal Two Black Carbon Pools Preserved in Large River Estuarine Sediments.

Black carbon (BC), a group of environmentally concentrated organic pollutants, is widely distributed in marine sediments via riverine run-off and atmospheric deposition. The fate of BC transformation and cycling in marine sediments, however, has not been well studied. Here, we present radiocarbon measurements for sedimentary solid-phase BC (SBC) and porewater-dissolved BC (DBC) in surface sediments collected from the Yangtze and Yellow River estuaries and their adjacent coastal regions. Radiocarbon results revealed that two distinct BC pools in the sediments of the SBC had ancient radiocarbon ages (7110-15,850 years BP) that were 5370-14,935 years older than the 14C ages of porewater DBC. Using a radiocarbon mass balance model, we calculated that modern biomass-derived BC contributed 77-97% of the DBC pool and that fossil material-produced BC accounted for 61-87% of the SBC pools. This discrepancy between modern and dead BC contributions was associated with the BC budget after particulate BC (PBC) deposition; 38 ± 13% of the PBC was transferred to porewater as DBC and 62 ± 13% was sequestrated as SBC in sediments, serving as an important CO2 sink in marine sediments. We also provide evidence suggesting that DBC likely comprises some very fine particulate forms that are not completely dissolved as molecules. The nature and transformation mechanisms of DBC in natural aquatic systems need to be further studied.

Borneo Vortices in a warmer climate

Borneo Vortices (BVs) are weather systems that are responsible for devastating hydro-climatic extremes and significant losses of life and property in Southeast Asia. The typical resolution of most current climate models is insufficient to resolve these high-impact, synoptic-scale weather systems. Here, an ensemble of high-resolution models projects that future BVs may become less frequent and more stationary, driven by the weakening of the Northeast monsoon flow and associated cold surges across North Borneo. However, substantial increases in both the intensity and the total amount of precipitation from BVs are projected. Such changes are driven by the more humid and convectively unstable lower troposphere. As a result, the contribution of BVs to the accumulation of both total precipitation and extreme precipitation is projected to increase considerably in the vicinity of the southern South China Sea, making individual BVs more threatening to the adjacent coastal regions.

LONG-TERM TREND OF NITROGEN AND PHOSPHORUS TRANSPORT IN 12 TROPICAL COASTAL WATERSHEDS IN NORTHEAST BRAZIL

Multiple environmental problems are caused by the accumulation of nitrogen (N) and phosphorus (P) in river water on its way to the ocean. The objective of this work was to determine the N and P loads in 12 hydrographic basins that flow into estuaries along the northeastern coast of Brazil using established methods. Additionally, natural and anthropogenic loads trends in the hydrographic basins of 12 tropical rivers in northeastern Brazil over 27 years (1990-2016) were evaluated. The results indicated an annual average increase of +1.1% in population density, while precipitation showed no significant trend. The anthropogenic loads were 21 and 112 times higher than natural inputs for N and P, respectively. Anthropogenic sources accounted for ~ 96% of all contributions. The positive trends of these loads (mainly wastewater) represented an annual average increase of 1.1% for N and 1.2% for P, for all river basins included in this study. These percentages represent average annual loads of 251 t N yr-1 and 54.8 t P yr-1. The time series of N and P in the study region showed trends that point to a constant increase, implying a greater contribution of these loads to the adjacent coastal region year after year.

Subsurface Eddy Facilitates Retention of Simulated Diel Vertical Migrators in a Biological Hotspot

AbstractDiel vertical migration (DVM) is common in zooplankton populations worldwide. Every day, zooplankton leave the productive surface ocean and migrate to deepwater to avoid visual predators and return to the surface at night to feed. This behavior may also help retain migrating zooplankton in biological hotspots. Compared to fast and variable surface currents, deep ocean currents are sluggish, and can be more consistent. The time spent in the subsurface layer is driven by day length and the depth of the surface mixed layer. A subsurface, recirculating eddy has recently been described in Palmer Deep Canyon (PDC), a submarine canyon in a biological hotspot located adjacent to the West Antarctic Peninsula. Circulation model simulations have shown that residence times of neutrally buoyant particles increase with depth within this feature. We hypothesize that DVM into the subsurface eddy increases local retention of migrating zooplankton in this feature and that shallow mixed layers and longer days increase residence times. We demonstrate that simulated vertically migrating zooplankton can have residence times on the order of 30 days over the canyon, which is five times greater than residence times of near‐surface, nonmigrating zooplankton within PDC and other adjacent coastal regions. The potential interaction of zooplankton with this subsurface feature may be important to the establishment of the biological hotspot around PDC by retaining food resources in the region. Acoustic field observations confirm the presence of vertical migrators in this region, suggesting that zooplankton retention due to the subsurface eddy is feasible.

Stark Physical and Biogeochemical Differences and Implications for Ecosystem Stressors in the Northeast Pacific Coastal Ocean

AbstractRegional differences in water properties and responses to stressors are key to understanding ecosystem vulnerabilities in coastal regions globally. Our study focuses on Canada's British Columbia coastal waters, from Queen Charlotte Strait to the Strait of Georgia. This area is bisected by shallow, narrow, tidally mixed channels with some of the strongest tidal currents in the world. We examine differences between regions on either side of this constriction, focusing on physical water properties and responses to three potential physical and biogeochemical stressors: nutrient levels, marine heatwaves, and ocean acidification. Our results quantify a spatially abrupt and temporally persistent lateral gradient in temperature, salinity, and density co‐located with a previously documented strong mixing zone. The distributions of density on either side of this front remain largely distinct throughout the tidal cycle, for all seasons, and for over 70 individual years for which data are available. Additionally, nutrient concentrations and molar ratios north of the front are statistically distinct from those to the south, and regions north of the front responded more strongly to the arrival of the marine heatwave known as “The Blob”. Lastly, regional differences in sensitivity to ocean acidification are identified, with high values at the surface in the Discovery Islands making that area very sensitive to ocean acidification. These results demonstrate the degree to which dramatic water property differences in adjacent coastal regions can persist over a wide range of time scales, with important connections to their responses to environmental stressors and implications for present and future ecosystem vulnerability.

Using Nd[sbnd]Sr isotopes in suspended sediments in the Abrolhos coral-reef (SW Atlantic, Brazil) to assess potential contamination from the 2015 Fundão dam collapse

The Abrolhos bank is home of the richest coral reef system of the Southwestern Atlantic, where endemic coral species are found. It has been reported that Abrolhos' corals are under intense stress due to increasing of Marine Heat Waves during the last decades. Additionally, anthropic interventions along the adjacent coastal regions are a factor of concern since they contribute to the increase in the sediment load and to organic debris input in the reef domain. In November 2015, the collapse of the Fundão mining tailings dam resulted in the release of approximately 50 million m3 of iron oxide and quartz-rich slurry into the Doce River. Aiming at using a fingerprint of the tailings and to assess the presence of traces of the Fundão dam material from this event on the Abrolhos bank, this work presents new 87Sr/86Sr and 143Nd/144Nd isotope ratios of marine suspended sediment samples collected between 2016 and 2020 from a network of sediment traps throughout the reef and complementary suspended material at sea. In parallel, we monitored meteo-oceanographic parameters and modeled surface marine currents as an attempt to identify the sediment transport between the Doce River mouth and Abrolhos bank. The r isotopes were used as provenance proxies based on the fact that minerals and rocks tend to have specific isotopic signatures reflecting their own geological derivation. In this context, the isotopic ratios of various potential regional sources for the sedimentation in Abrolhos bank were evaluated. Our monitoring and isotopic measurements indicate that Doce River signatures are detected at Abrolhos bank, following the seasonal Doce River discharge at sea. Isotopic signature of Doce River at Abrolhos bank was also observed during the austral winter (July–August) when cold fronts migrate at the Brazilian coast with higher frequency and energy.

Evaluation of methods for connecting InSAR to a terrestrial reference frame in the Latrobe Valley, Australia

Deformation measurements from satellite-borne synthetic aperture radar interferometry (InSAR) are usually measured relative to an arbitrary reference point (RP) of assumed stability over time. For InSAR rates to be reliably interpreted as uplift or subsidence, they must be connected to a defined Earth-centred terrestrial reference frame (TRF), usually made through GNSS continuously operating reference stations (CORS). We adapt and compare three methods of TRF connection proposed by different studies which we term the single CORS RP (SCRP), plane-fit multiple CORS (PFMC), and the multiple CORS RP (MCRP). We generalise equations for these methods, and importantly, develop equations to propagate InSAR and GNSS uncertainties through the transformation process. This is significant, because it is important to not only estimate the InSAR uncertainties, but also to account for the uncertainties that are introduced when connecting to the CORS so as to better inform our interpretation of the deformation field and the limitation of the measurements. We then test these methods using Sentinel-1 data in the Latrobe Valley, Australia. These results indicate that differences among the three TRF connection methods may be greater than their estimated uncertainties. MCRP appears the most reliable method, although it may be limited in large study areas with sparse CORS due to long wavelength InSAR errors and that gaps and/or steps may appear at the spatial limit from the CORS. SCRP relies on the quality of the single CORS connection, but can be validated by unconnected CORS in the study area. The PFMC method is suited to larger areas undergoing slow, constant deformation covering large spatial extents where there are evenly distributed CORS across the study area. Selecting an optimal method of TRF connection is dependent on local site conditions, CORS network geometry and the characteristics of the deformation field. Hence, the choice of TRF connection method should be carefully considered, because different methods may result in significantly different transformed deformation rates. We confirm slow subsidence across the Latrobe Valley relative to the vertical component of the ITRF2014, with localised high subsidence rates near open cut mining activities. Subsidence of ~ -6 mm/year is observed in the adjacent coastal region which may exacerbate relative sea level rise along the coastline, increasing future risks of coastal inundation.

A NEW SPECIES OF CRYPTANTHA RESTRICTED TO DUNES IN NORTHWESTERN BAJA CALIFORNIA, MEXICO

Cryptantha arenophila Rebman & M.G.Simpson (Boraginaceae) is described as new. This species is restricted to sandy dunes near San Quintín, Baja California, Mexico and adjacent coastal regions. It is similar to the more common Cryptantha patula Greene in stem vestiture, calyx morphology, and inflorescence morphology. Cryptantha arenophila differs from that species in having significantly larger corolla limbs and narrowly oblong to narrowly elliptic leaves with obtuse-rounded leaf apices, as opposed to linear to narrowly lanceolate leaves with acute to obtuse leaf apices in C. patula. In addition, nutlets of C. arenophila tend to have more numerous and shorter tubercles, while those of C. patula have tubercles that are less dense and larger, although variation in this species needs further investigation. The dune habitats where the type locality of C. arenophila occurs are currently under severe impacts from animal grazing and off-road traffic, necessitating the conservation of these regions.

Mapping of Coastline Changes in Athens Riviera over the Past 76 Year’s Measurements

Mapping of coastline changes assists in coastal development and monitoring. Athens Riviera, located in the southwestern coastal zone in Attica in central Greece, has undergone major and radical changes generated by human interference during the last decades. The aim of this paper is to map record and measure the coastline changes in Athens Riviera over the past 76 years. Aerial photographs and satellite images from 1945 to 2021 as well as geographic information system (GIS) techniques were used to depict the spatial and temporal variations of the coastline. The results show that 60% of the total length of the coastline is artificial coast, while 29% is rocky coast and 12% beach. The study of seven subareas showed that human interventions caused significant coastline changes in Faliro Bay, Alimos and Glyfada. The adjacent coastal regions to Athens metropolitan area illustrate the highest modifications in the coastline. The main changes in the coastlines are recorded from 1960 to 1987, while the rate of changes in coastline slowed down from 1987 to 2021. The total changes in the coastline of Athens Riviera demonstrate that, during the past 76 years, the coastline was enhanced by 40% while land reclaimed to the sea area approaches 2.67 km2. The applied method is effective and rapid and may utilize in the coastal monitoring and management.

Organic carbon production and nutrients consumption revealed by mixing model in the energetic Changjiang Estuary

Riverine and oceanic nutrients co-influence the estuarine and coastal phytoplankton production and subsequent organic carbon (OC) dynamics, which contribute significantly to the global carbon cycle. However, it is difficult to quantify the contributions of nutrients from different sources. Based on in situ observations made in July 2017, a three end-member model was developed to estimate the nutrients consumed and OC produced by biological activities in the Changjiang Estuary and its adjacent coastal region. In this way, the contributions of nutrients from different sources to phytoplankton production were quantified. In onshore water with salinity less than 20, the nutrients consumed and OC produced were mainly controlled by the mixing of various water masses and turbidity maximum zones. In offshore water with salinity larger than 20, the model results suggested that the biogeochemical processes regulating nutrients and OC dynamics were mainly impacted by the Changjiang River plume and Taiwan Warm Current deep water (TWCDW). In the stratified river plume region, nutrient uptake and OC production were controlled by the contributions of three end-members (Changjiang fresh water, Taiwan Warm Current surface water and TWCDW), while in the upwelling-influenced region, OC production was strongly regulated by the nutrients supplied by the TWCDW. This study will also shed light on the influence of multi-sourced nutrient compositions and structures on the in situ biological OC production process in similar dynamic estuarine regions.

Spatial and temporal variations in dissolved silicate along the Indian coastal groundwaters and their export to adjacent coastal waters

Silicate plays a vital role in the coastal regions biogeochemistry. Submarine Groundwater Discharge (SGD) is one of the significant silicate sources to the coastal regions. The silicate concentrations in groundwater (GW) are considerably higher than in neighbouring coastal waters, as a result, SGD increases the concentrations of silicate in the coastal waters and influences the marine ecosystem. To examine the spatial and temporal variability in silicate concentrations and export of silicate to the adjacent coastal waters through SGD, groundwater samples are collected at 90 locations along the east and west coast regions of India during the dry and wet periods. This study revealed that the concentration of dissolved silicate was lower during the dry period than during the wet period. The soil silicate concentration displayed a significant variability and has a good correlation with silicate concentrations in the coastal groundwater of India. Silicate in groundwater showed significant linear relations with bacterial respiration, bicarbonate, and pCO2 and inverse relation with dissolved oxygen saturation suggesting that remineralization and weathering processes are responsible for the silicate in the coastal groundwater of India. The average SGD of freshwater varied between 1.6 × 104 m3/day and 1.75 × 1011 m3/day from the Indian coastal groundwater into adjacent coastal regions. The fluxes of silicate through SGD to the Bay of Bengal was estimated as 1.97 ± 0.85 Tg (1 Tg = 1012 g) and 3.22 ± 1.56 Tg to the eastern Arabian Sea. The estimated total SGD fluxes of silicate to the northern Indian ocean from east and west coastal regions of India is 3.07 ± 1.15 Tg/y and it is almost three times higher than silicate discharged from monsoonal rivers 1.03 ± 0.25 Tg/y. The silicate through SGD from Indian coastal groundwater contributes 7% of silicate to the total silicate export to the world coastal regions through SGD.

Model-Based Evaluation of Hydroelectric Dam’s Impact on the Seasonal Variabilities of POC in Coastal Ocean: A Case Study of Three Gorges Project

Particulate organic carbon (POC) plays an important role in the global carbon cycle. The POC in the Changjiang Estuary and adjacent coastal region of the East China Sea (ECS) is dominated by riverine input and marine production and is significantly influenced by the three gorges project (TGP). A coupled physical–biogeochemical model was used to evaluate TGP’s impact on POC. The results demonstrate that TGP regulates the area influenced by diluted water and POC through direct river and sediment discharge and affects the ecosystem. From the early to later TGP construction periods, the surface region with high-POC concentration (>40 μmol L−1) decreases by 20.5% in area and 11.5% in concentration. Meanwhile, POC in the whole water column decreases from 19.5 to 17.8 μmol L−1. By contrast, the concentrations of chlorophyll-a (Chl-a) and related nutrients increase. A three end-member mixing model based on quasi-conservative temperature and salinity is used to quantify relative contributions of different water sources to POC in our research area. We also estimate the biological POC production by the difference between the physical-biogeochemical model predicted POC and three end-member model mixing POC. The result demonstrate that under the regulation of TGP in the later period, the decrease of sediment load increases water transparency, which favors photosynthesis and oceanic biological produced POC. In addition, over 70% of the areas have C/Chl-a > 200 and high C/N ratios, which are circumstantial evidences that organic detritus and terrestrial input sources still dominate in the Changjiang Estuary and adjacent coastal ECS but are influenced by TGP’s regulation.

Warming from loss

Arctic Sea Ice Arctic sea ice is disappearing at an alarming rate, a situation with many potential consequences including an acceleration of warming due to a decrease in albedo. Pedersen and Christensen used a suite of climate models to study how sea ice loss in various regions of the Arctic affects surface temperatures across Greenland. They found that ice loss near Greenland has the strongest warming effect on adjacent coastal regions but little impact on the interior of the ice sheet. Sea ice loss in the remote central Arctic has no significant influence on central Greenland. So, although sea ice loss can be the major cause of warming in coastal areas, the bulk of Greenland is much more sensitive to global factors. Geophys. Res. Lett. 10.1029/2019GL083828 (2019).

SST Indexes in the Tropical South Atlantic for Forecasting Rainy Seasons in Northeast Brazil

May-to-July and February-to-April represent peak rainy seasons in two sub-regions of Northeast Brazil (NEB): Eastern NEB and Northern NEB respectively. In this paper, we identify key oceanic indexes in the tropical South Atlantic for driving these two rainy seasons. In Eastern NEB, the May-to-July rainfall anomalies present a positive relationship with the previous boreal winter sea surface temperature anomalies (SSTA) in the southeast tropical Atlantic (20°–10° S; 10° W–5° E). This positive relationship, which spread westward along the southern branch of the South Equatorial Current, is associated with northwesterly surface wind anomalies. A warmer sea surface temperature in the southwestern Atlantic warm pool increases the moisture flux convergence, as well as its ascending motion and, hence, the rainfall along the adjacent coastal region. For the Northern NEB, another positive relationship is observed between the February-to-April rainfall anomalies and the SSTA of the previous boreal summer in the Atlantic Niño region (3° S–3° N; 20° W–0°). The negative remote relationship noticeable between the Northern NEB rainfall and the concomitant Pacific Niño/Niña follows cold/warm events occurring during the previous boreal summer in the eastern equatorial Atlantic. The southeastern tropical Atlantic and Atlantic Niño SSTA indexes may, then, be useful to predict seasonal rainfall over the Eastern and Northern NEB, respectively, for about a 6 month leading period. The ability of both southeastern tropical Atlantic and Atlantic Niño SSTA indexes to forecast the Eastern and Northern NEB rainfall, with about a 6 month lead time, is improved when these indexes are respectively combined with the Niño3 (5° S–5° N; 150°–90° W) and the northeast subtropical Atlantic (20° N–35° N, 45° W–20° W), mainly from the 1970’s climate shift.

Response of tropical cyclone Phailin (2013) in the Bay of Bengal to climate perturbations

Tropical cyclones in the Bay of Bengal (BoB) are the major source of loss of properties and lives in the adjacent coastal regions during post-monsoon (October–November) season. This study explores the impact of large-scale environmental change on tropical cyclone (TC) Phailin (2013) in the Bay of Bengal. An ensemble simulation with a regional atmospheric model using different physics and initial conditions is able to capture the observed track of Phailin. The cyclone intensity, however, is underestimated due to the coarse resolution (10 km) of the model. To explore Phailin’s response to a warming climate, a global climate model change signal is added to the cyclone environment using the pseudo-global warming approach, and the ensemble is run again. It is found that changes in the track of Phailin would be small, except around the time of landfall, when Phailin shifts southward in its track. The cyclone becomes more intense and larger, but the translation speed remains nearly unchanged. An analysis of physical processes indicates a deeper TC core due to enhanced latent heating. The vertical distribution of temperature and wind indicates that the eye of Phailin would have a greater vertical extent in the future climate. The expanded size is found to be due to an increase in environmental humidity and temperature in the lower-troposphere leading to an increase in environmental convective available potential energy. The translation speed remains nearly unchanged due to insignificant changes in the steering flow. The result suggests that Phailin, under a warmer climate, may have higher damage potential (~ 22%) compared to the present at the time of landfall.

Intertidal regions changing coastal alkalinity: The Wadden Sea‐North Sea tidally coupled bioreactor

AbstractIn this study, we successfully implemented a total alkalinity (TA) analyzer in a flow‐through setup, in combination with a FerryBox. The high‐frequency (10 min) measurements along our ship's route revealed that in coastal systems, where carbon fluxes are dynamic, TA can differ significantly (by up to 100 μmol kg−1) between the nearshore and adjacent coastal regions. Even though this study could not account for the net yearly TA production in the coastal region, it demonstrated that there was a seasonal increase in TA of 100–150 μmol kg−1 in coastal waters of the North Sea, equivalent to TA production of 11.7–26.8 mmol m−2 d−1 during the spring and summer months. This seasonal change could not be accounted for by riverine contributions, but instead was probably related to seasonal organic matter production and processing in coastal and nearshore regions. Bottom sediments and the tidally coupled biogeochemical reactor between coastal (North Sea) and nearshore (Wadden Sea) regions are mediating this TA change, and the ~ 4 months lag between the seasonal increase in alkalinity and the peak organic matter production could be explained by the supply of (labile) organic matter and its temperature‐dependent remineralization via both aerobic and anaerobic pathways.

High bacterial activity in nutrient rich saltwater: Evidence from the uncoupling between salinity and nutrients in the Patos Lagoon estuary

In estuaries, a common pattern found is of higher bacterial activity in freshwater than in saltwater, an indication of the existence of an activity gradient. It is not clear, however, whether this arises from a natural property of saltwater bacterial communities or from metabolic constraints imposed from the natural co-variation between salinity and nutrients in estuarine systems. In this study, we managed to detect in situ higher activity levels of free-living bacteria in nutrient-rich saltwater entering the Patos Lagoon estuary. The bacterial community composition (BCC - Temporal Temperature Gradient Gel Electrophoresis) and activity (community-level physiological profiles, CLPP- Ecoplate™ - Biolog) were evaluated during two days in a transect along the estuary until the adjacent coastal region. The unexpected higher nutrient content and bacterial activity in saltwater found suggest that the bacterial activity is a primary response to nutrient availability, whereas salinity is the main factor structuring BCC in this estuary. Our results reject the idea of a natural low activity of saltwater bacteria by showing that their activity can increase in situ given that the proper conditions are met, especially in estuaries that display circulation patterns that promote the occurrence of nutrient-rich saltwater.