Mesotidal Coastal Lagoon Research Articles

Impact of Extreme Wind and Freshwater Runoff on the Salinity Patterns of a Mesotidal Coastal Lagoon

The interaction between tide, river runoff, and wind in coastal lagoons induces complex salinity gradients, which are remarkable when the meteorological forcing is exacerbated. This work aims to characterize the salinity structure under extreme freshwater and wind events in the Ria de Aveiro coastal lagoon (Portugal). The Delft3D model was implemented and validated in 3D mode and used to perform simulations forced with extreme freshwater and wind scenarios. Results show that forcing conditions determine salinity stratification intensity and location. Generally, stratification increases as the freshwater increases, while the salinity intrusion moves downstream. Extreme wind tends to destroy stratification but fails to promote full-depth mixing, which is also dependent on the wind direction, as shown for the Espinheiro channel. The salinity intrusion is also impacted by wind events, being NW storms responsible for an upstream salt transport along the Mira channel and a downstream transport along the Espinheiro channel, and SW storms for an upstream displacement of the salinity intrusion along the São Jacinto channel. Finally, it is observed that the advection of a freshwater plume from the Vouga River into the middle of the São Jacinto channel under high freshwater scenarios causes an unusual local salinity pattern. This plume can either be pushed upstream or prevented from entering the channel, depending on the wind direction.

Adapting a dynamic system model using life traits and local fishery knowledge — Application to a population of exploited marine bivalves (Ruditapes philippinarum) in a mesotidal coastal lagoon

Effective planning and management strategies of the Manila clam (Ruditapes philippinarum) stock in Arcachon Bay rely on a co-management approach, involving scientific advice and the fishing industry. In order to strengthen this management process, a specific compartmental single-species model has been developed to assess different management strategies. Based upon a size structure of the population and built from the dynamic systems paradigm, the modelling involves selected environmental and fishery-related parameters for their impact on the population dynamics. Integration of newly available life traits data regarding the local population and better knowledge of the fishing activity and its control parameters have enabled a revision of the existing version including model parameters and integration of new formulations. Effect of food availability and total suspended matter on the production of newly settled juveniles, natural mortality, growth rates, and price, accounted for the bulk of the undertaken improvements. Model performance was evaluated by comparing simulation outputs to observed data from stock assessments (2003, 2006, 2008, 2010, 2012, 2014 and 2018). In addition, a sensitivity analysis regarding uncertainty on parameters related to biological and environmental aspects was carried out to provide information about parameters that influence the outputs of the model the most. In particular, this highlighted that a deeper understanding of the processes related to the reproduction phase and its control parameters on biomass should be pursued. The improvements enable the calibration of the model with parameters that are as close as possible to the specificities of the stock considered. This adaptation permits use of the model to test various management strategies on the stock evolution and also to evaluate potential effects of environmental or market fluctuations. It encompasses ecological and socio-economic valuations of the measures of impact. As long as those objectives are shared by the different entities involved at the regional scale, communication and management decisions should be facilitated.

Vertical intertidal variation of organic matter stocks and patterns of sediment deposition in a mesotidal coastal wetland

Tidal coastal wetlands, common home to seagrass and salt marshes, are relevant carbon sinks due to their high capacity to accumulate and store organic carbon in their sediments. Recent studies demonstrated that the spatial variability of this organic carbon within the same wetland system can be significant. Some of the environmental drivers of this spatial variability remain understudied and the selection of the most relevant ones can be context dependent. Here we investigated the role of bed elevation, hydrodynamics, and habitat type (salt marsh and seagrass) on the organic matter (OM) net deposition-resuspension rate and superficial sedimentary stocks (top 5 cm) at the tidal wetlands of the Ria Formosa, a mesotidal coastal lagoon in South Portugal. Results showed that two vectors of spatial variation need to be considered to describe the intertidal sedimentary OM stocks: the bed elevation that imposes a decrease of the hydroperiod and thus the change of habitat from the lower seagrass Z. noltei to the upper saltmarsh S. maritimus, and the horizontal spatial variation along the secondary channels of the lagoon that imposes a decrease in the current flow velocity magnitude. The multiple linear regression analyses, using data from 40 sampling points, explained 59% of the variation of the superficial sedimentary stocks of OM in salt marshes and seagrasses of the Ria Formosa lagoon and revealed that stocks generally decrease with elevation, yet with variation among sites and habitats. It was also found that the decrease of the OM net deposition-resuspension rate with bed elevation was exponential. Our study emphasizes the importance of considering multiple environmental drivers and spatial variation for regional estimations of organic matter (and organic carbon) sedimentary stocks in coastal wetlands.

Carbon and Nitrogen Stocks and Burial Rates in Intertidal Vegetated Habitats of a Mesotidal Coastal Lagoon

Coastal vegetated ecosystems such as saltmarshes and seagrasses are important sinks of organic carbon (OC) and total nitrogen (TN), with large global and local variability, driven by the confluence of many physical and ecological factors. Here we show that sedimentary OC and TN stocks of intertidal saltmarsh (Sporobolus maritimus) and seagrass (Zostera noltei) habitats increased between two- and fourfold along a decreasing flow velocity gradient in Ria Formosa lagoon (south Portugal). A similar twofold increase was also observed for OC and TN burial rates of S. maritimus and of almost one order of magnitude for Z. noltei. Stable isotope mixing models identify allochthonous particulate organic matter as the main source to the sedimentary pools in both habitats (39–68%). This is the second estimate of OC stocks and the first of OC burial rates in Z. noltei, a small, fast-growing species that is widely distributed in Europe (41,000 ha) and which area is presently expanding (8600 ha in 2000s). Its wide range of OC stocks (29–99 Mg ha−1) and burial rates (15–122 g m2 y−1) observed in Ria Formosa highlight the importance of investigating the drivers of such variability to develop global blue carbon models. The TN stocks (7–11 Mg ha−1) and burial rates (2–4 g m−2 y−1) of Z. noltei were generally higher than seagrasses elsewhere. The OC and TN stocks (29–101 and 3–11 Mg ha−1, respectively) and burial rates (19–39 and 3–5 g m−2 y−1) in S. maritimus saltmarshes are generally lower than those located in estuaries subjected to larger accumulation of terrestrial organic matter.

New insights about the primary production dependence on abiotic factors: Ria de Aveiro case study

Abstract The Ria de Aveiro is a mesotidal coastal lagoon located in the north coast of Portugal. Although the biophysics of this coastal system has been extensively researched in previous studies, there is still a lack of knowledge about the fate of biomass of fluvial origin in the lagoon and on the on the limiting factors of growth of their plankton communities. Based on this assumption, this study presents a first attempt to perform an integrated assessment of the temporal and spatial evolution of water temperature, salinity, pH and chlorophyll a concentration in Ria de Aveiro lagoon, incorporating analysis of in situ data and numerical model predictions of local biophysics. Additionally, the influence of light availability, nutrient limitation and water temperature in lagoon’s phytoplankton biomass was also evaluated. Water temperature, salinity, pH and chlorophyll a were measured in water samples collected monthly from February 2016 until February 2017, in 12 stations strategically located along the lagoon main channels. Chlorophyll a concentration was measured by extraction with aqueous acetone and quantified spectrophotometrycally. A model implementation based on Delft 3D suite was developed to simulate Ria de Aveiro hydrodynamics and water quality, and one-year simulation was performed including the sampling period. The results show a seasonal and longitudinal variability of the biophysical variables under analysis, allowing its quantification under different conditions in such a complex system. Particularly, the phytoplankton biomass showed a typical seasonal variability, characterized by a spring/summer bloom with a maximum of 8 μg/L, when environmental conditions are suitable to their growth, in opposition to nutrients concentration, that shows a 90% uptake of the total available by the phytoplankton communities. During these periods, Ria de Aveiro lagoon was found to be nutrient limited. Despite the seasonal light limitation, the primary production is strongly ruled by water temperature and nutrients concentration variability. These are strongly dependent on the riverine input, which is the most important natural source controlling the water temperature and nutrients concentration variability. The spatial and temporal distribution of the different variables is strongly influenced by the tidal dynamics, but also by geomorphologic features, such as depth and narrowness of the channels, which play therefore a major role in water quality characteristics. In summary, the water temperature establishes the seasonal variability, limiting photosynthesis in winter to values close to 0, while nutrients concentrations rule the primary production during spring and summer, since the nutrients uptake by phytoplankton and other marine organisms is about 90% higher than the replenishment from oceanic and riverine origin. These conclusions upgrade the state of the art about local processes, which generally referred that plankton community’s growth in Ria de Aveiro is nutrients limited.

Evaluation of macrobenthic community responses to dredging through a multimetric approach: Effective or apparent recovery?

Macrobenthic responses to dredging activities were evaluated in a mesotidal coastal lagoon (Ria Formosa – South Portugal). Four areas (2 dredged and 2 non-dredged) were analysed along different dredging situations (pre-dredging, during dredge 1, during dredge 2 and post-dredging). Beyond the evaluation of sediment descriptors and classical ecological indicators (number of species and abundance), the beta diversity component (partitioned into turnover and nestedness) was used to detect dredging effects on macrobenthic communities, together with a multivariate analysis (both quantitative and qualitative) including the ecological status assessment. Results confirmed the limitations of alpha diversity indicators in detecting dredging effects in naturally stressed systems, given the absence of clear patterns regarding number of species, abundance and equitability among areas and dredging situations. Moreover, post-dredging presented a higher number of species compared to pre-dredging, suggesting a positive disequilibrium induced by these activities. In contrast, partitioned beta diversity analysis revealed higher nestedness values at dredged areas, while turnover clearly prevailed at non-dredged ones. Such results suggest an effective impact of dredging on these communities, namely through the decrease of rarer species after sediment removal. This qualitative analysis, based on presence-absence data (Sorensen dissimilarity), was complemented by multivariate analysis (SIMPER and PERMANOVA) based on quantitative data (Bray-Curtis dissimilarity), highlighting the importance of adopting both approaches to detect dredging impacts on benthic communities. Lower effectiveness was found for M-AMBI index when evaluating benthic Ecological Status, since the apparent absence of organic matter and sediment composition shifts induced by dredging, together with the noticeable high diversity even during dredging situations, strongly limited the analysis. The multimetric approach here proposed can contribute to detect dredging effects on lagoonal benthic communities by disentangling the responses caused by dredging from those resulting from the system natural dynamics.

Sediment dynamics of an artificially deepened mesotidal coastal lagoon: An environmental magnetic investigation of Tauranga Harbour, New Zealand

Tauranga Harbour, New Zealand's largest barrier-enclosed coastal lagoon, comprises two sub-basins with separate catchments, inlets and tidal channel systems. This study sets out to assess and investigate the sediment dispersal system of Tauranga Harbour using standard environmental magnetic and sedimentological methods. Compilations of rock magnetic and grain size data of surficial sediments collected from inflowing rivers, various estuarine environments (estuaries, tidal flats and tidal channels) and the adjacent nearshore mirror the net and differential sediment fluxes into and through the two sub-basins of this lagoon. For all studied depositional environments, the magnetogranulometric ratios SIRM/κ and ARM/κ are positively correlated with magnetic mineral content (SIRM, ARM, χ) in the sense that larger magnetic particles are associated with higher magnetic enrichment. Grain-size analyses show that magnetic enrichment during particle transport and deposition can result from grain-size as well as from grain-density fractionation. The periodically changing accumulation/erosion conditions provide for a selective retention of specific grain sizes and an enhancement of the heavier magnetic mineral particles. Magnetic crystal size and clastic grain size correlate poorly over the whole study area, but group within similar depositional environments. Coarser magnetic and clastic grain sizes and higher magnetite enrichment in the southern sub-basin can be reconciled with episodic flood runoff of the Wairoa River and a much larger, artificially deepened southern tidal inlet which likely enables higher tidal current velocities. Our approach of combining magnetic and clastic grain-size could be successfully implemented to establish a conceptual model of sediment dynamics and gravitational sorting within Tauranga Harbour.

\u2018Blue Carbon\u2019 and Nutrient Stocks of Salt Marshes at a Temperate Coastal Lagoon (Ria de Aveiro, Portugal)

Ria de Aveiro is a mesotidal coastal lagoon with one of the largest continuous salt marshes in Europe. The objective of this work was to assess C, N and P stocks of Spartina maritima (low marsh pioneer halophyte) and Juncus maritimus (representative of mid-high marsh halophytes) combined with the contribution of Halimione portulacoides, Sarcocornia perennis, and Bolbochenous maritimus to the lagoon ≈4400 ha marsh area. A multivariate analysis (PCO), taking into account environmental variables and the annual biomass and nutrient dynamics, showed that there are no clear seasonal or spatial differences within low or mid-high marshes, but clearly separates J. maritimus and S. maritima marshes. Calculations of C, N and P stocks in the biomass of the five most representative halophytes plus the respective rhizosediment (25 cm depth), and taking into account their relative coverage, represents 252053 Mg C, 38100 Mg N and 7563 Mg P. Over 90% of the stocks are found within mid-high marshes. This work shows the importance of this lagoon’s salt marshes on climate and nutrients regulation, and defines the current condition concerning the ‘blue carbon’ and nutrient stocks, as a basis for prospective future scenarios of salt marsh degradation or loss, namely under SLR context.

Primary production of the benthic microalgae in the bottom sediments of Ria de Aveiro lagoon

ABSTRACT Vaz, L.; Serodio, J., Mateus, M., Dias, J.M., and Vaz, N., 2016. Primary production of the benthic microalgae in the bottom sediments of Ria de Aveiro Lagoon. In: Vila-Concejo, A.; Bruce, E.; Kennedy, D.M., and McCarroll, R.J. (eds.), Proceedings of the 14th International Coastal Symposium (Sydney, Australia). Journal of Coastal Research, Special Issue, No. 75, pp. 178–182. Coconut Creek (Florida), ISSN 0749-0208. The Ria de Aveiro is a mesotidal coastal lagoon located in the north coast of Portugal. It has a complex irregular geometry characterized by large intertidal areas and several freshwater tributaries, which are the main sources of nutrients into the lagoon. The dynamics between the tidal propagation and the landward nutrient sources modulates the primary production within the lagoon. Here, primary production may have two main contributors: the chlorophyll in the water column and the benthic microalgae in the bottom sediments. In this work, a new methodology to compute microalgae in the b...

Nutrients and clam contamination by Escherichia coli in a meso-tidal coastal lagoon: Seasonal variation in counter cycle to external sources

The clam Ruditapes decussatus was transplanted from a natural recruitment area of Ria Formosa to three sites, surveyed for nutrients in water and sediments. Specimens were sampled monthly for determination of Escherichia coli, condition index and gonadal index. Higher nutrient values in low tide reflect drainage, anthropogenic sources or sediment regeneration, emphasising the importance of water mixing in the entire lagoon driven by the tide. Despite the increase of effluent discharges in summer due to tourism, nutrient concentrations and E. coli in clams were lower in warmer periods. The bactericide effect of temperature and solar radiation was better defined in clams from the inlet channel site than from sites closer to urban effluents. High temperature in summer and torrential freshwater inputs to Ria Formosa may anticipate climate change scenarios for south Europe. Seasonal variation of nutrients and clam contamination may thus point to possible alterations in coastal lagoons and their ecosystem services.

A new species of Onuphis (Polychaeta: Onuphidae) from Southern Portugal, with comments on the validity of O. pancerii Claparède, 1868.

Onuphis farensis sp. nov. (Annelida, Polychaeta) is described, based on a population inhabiting intertidal sandbanks in the mesotidal coastal lagoon of Ria Formosa (Southern Portugal). It can be distinguished from all other known species within the genus by having bi- and tridentate pseudocompound hooks on the first 4 chaetigers, single filament branchiae from chaetiger 5, and subacicular hooks from chaetiger 9. The species was previously collected in the Bay of Cádiz and Isla Cristina (SW Spain), in a similar habitat to Ria Formosa, but referred to O. geophiliformis Moore, 1903. The taxonomic status of other Onuphis species recorded in the European waters is also discussed. On the whole, O. pancerii Claparède, 1868, described from the Gulf of Naples, has been treated as a junior synonym of O. eremita Audouin & Milne-Edwards, 1833, but taking the more restrictive definition of this species accepted today, should be considered as a valid Mediterranean species. Finally, O. opalina (Verrill, 1873) and O. rullieriana (Amoureux, 1977) may be synonymous, as both species are similar morphologically, occur at similar depths, and have partially overlapping geographical distributions. The different diagnostic characters utilised for the new species are analysed, with the number of chaetigers with postchaetal lobes determined to be a poor taxonomic character for the genus Onuphis, proving to be size-related. A synoptic table with all worldwide species of the genus Onuphis is provided, together with a dichotomic key for the species hitherto recorded in the European and nearby waters.

Salinity modelling accuracy of a coastal lagoon: a comparative river flow analysis of basin model vs. traditional approaches

ABSTRACT Tomas, L.M., Rodrigues, M, Fortunato, A.B., Azevedo, A, Leitao, P.C., Oliveira, A., Rocha, A., Lopes, J.F., Dias, J.M., 2014. Salinity modelling accuracy of a coastal lagoon: a comparative river flow analysis of basin model vs. traditional approaches. In: Green, A.N. and Cooper, J.A.G. (eds.), Proceedings 13th International Coastal Symposium (Durban, South Africa), Journal of Coastal Research, Special Issue No. 70, pp. 586–591, ISSN 0749-0208.. The main purpose of this study is to investigate the uncertainties in the modelling of salinity fields in the Ria de Aveiro lagoon associated with the estimation of river flow discharges. The prediction of fresh water inputs is necessary to properly implement forcing conditions and consequently to provide accurate forecasts of baroclinic circulation in coastal lagoons. Located in the north-western Portuguese coast, the Ria de Aveiro is a shallow vertically homogeneous mesotidal coastal lagoon with a complex geometry. Although it is tidally dominated, it re...

Ecosystem functioning: The basis for restoration and management of a tropical coastal lagoon, Pacific coast of Mexico

Tropical coastal lagoons (TCL) are among the most productive coastal ecosystems, and they provide a wide range of ecosystem goods and services that support productive economic activities. The objective of this paper is to present a comprehensive review of the ecosystem dynamics in Paraman Lagoon, a wave-dominated, meso-tidal coastal lagoon with an ephemeral inlet on the Pacific coast of Mexico. A conceptual model is used to describe the seasonal environmental cycle for the lagoon, which is characterized by three periods: Period 1 - disconnected from the sea, evaporation is much higher than freshwater input and salinity can exceed 75psu; Period 2 - disconnected from the sea, freshwater input is much higher than evaporation and salinity drops to as low as 5psu; and Period 3 - the inlet is open connecting the lagoon with the ocean resulting in optimal estuarine conditions and moderate salinities between 15 to 25psu. Hydrology and salinity regime in the lagoon are largely controlled by the amount of freshwater discharge that changes the spatial and temporal distribution of environmental parameters. This information is used to develop a conceptual model of the functioning of the lagoon that can be used to aid in decision-making about the restoration and management of the lagoon

Seasonal Variations in Gross Biochemical Composition, Percent Edibility, and Condition Index of the ClamRuditapes decussatusCultivated in the Ria Formosa (South Portugal)

The grooved carpet shell clam Ruditapes decussatus (L. 1758) is one of the most popular and profitable molluscs exploited in rearing plots in the Mediterranean. However, annual catch has been declining steadily since the early 1990s. To understand the seasonality of its nutritional value, thus providing an improved basis for economic valuation of the resource, gross biochemical composition, percent edibility, and condition index were investigated during a year with monthly periodicity in a commercially exploited population of the clam R. decussatus in the Ria Formosa, a temperate mesotidal coastal lagoon located in the south of Portugal. Our results show that total and nonprotein nitrogen covaried during the year, resulting in a protein content that peaked in the warmest months. Although complementary in summer, carbohydrate and lipid contents showed irregular annual trends. The observed seasonality was comparable with that shown by studies elsewhere at similar latitudes, and is underpinned by the reproductive cycle of the species. Our results show the clams to be at their prime nutritional value at the beginning of summer, when protein content peaks.

The effect of benthic sediments on dissolved nutrient concentrations and fluxes

The Ria Formosa is a meso-tidal coastal lagoon experiencing enhanced nutrient concentrations. Assessment of sediment–seawater interaction is essential if nutrient dynamics and the risk of eutrophication are to be fully understood. Pore water concentrations of dissolved inorganic and organic phosphorus, ammonium, nitrate and nitrite were determined in cores from six sites. Changes in nutrients concentrations were measured in intertidal pools on sand and mud between tides. Dissolved inorganic phosphorus (DIP) concentrations (~200 µmol l−1) and effluxes (123 ± 14 µmol m−2 h−1) were greater from sand than mud (37 ± 10 µmol m−2 h−1), possibly due to the binding of P with the <63 µm fraction. NH4+ effluxes were high outside the Ancao Basin (821 ± 106 µmol m−2 h−1) and were associated with Enteromorpha sp. mats. The greatest NO3− efflux was from sediments near a salt marsh (170 ± 67 µmol m−2 h−1). These sediment fluxes of P were not sufficient to account for elevated P concentrations seen by other workers on the ebb tide from the Ancao Basin. Intertidal pools were sinks for Dissolved Inorganic Nitrogen (DIN) and DIP over the 6 h exposure period. Thus, tidepools may be an important route of nutrients into sediments that enhances the effects of sediments on seawater nutrient concentrations.

Nutrient dynamics in a coastal lagoon (Ria Formosa, Portugal): The importance of lagoon–sea water exchanges on the biological productivity

Silicates, nitrates, phosphates, chlorophyll a and primary productivity were monitored at low and high tide between September 1985 and September 1986, at four stations in Ria Formosa, a meso-tidal coastal lagoon located in the south of Portugal. The water sampling and the in situ observations were done alternatively in neap and spring tide. While silicates at low tide exceeded values at high tide thorughout the year and a weak seasonal trend was discerned, nitrates and phosphates exhibited a clear-cut seasonal variation at high spring tide. Pronounced maxima in the period of low temperatures (early spring), 7 µM of nitrates and 3 µM of phosphates, indicate their import from the adjacent coastal zone. Lower values at low tide and chlorophyll a increases (up to 2 µg g–1) point to a rapid biological consumption of the imported nutrients. Differences of temperature between coastal waters and those of the lagoon in early spring account for the more efficient consumption by the primary producers of the lagoon. The primary productivity varied from 2 mg C m–3 h–1 (winter) to 30 mg C m–3 h–1 (summer), reflecting the intensive 14C incorporation during the warmer period. High rates of nitrates and phosphates consumption could be observed during spring, when their concentrations decreased 50% within a narrow period of 6 hours from high to low tide.

The Quantitative Distribution of Subtidal Macrozoobenthic Assemblages in Arcachon Bay in Relation to Environmental Factors: a Multivariate Analysis

The species composition and spatial distribution of macrozoobenthic assemblages in the subtidal, unvegetated areas of Arcachon Bay (a mesotidal coastal lagoon on the SW coast of France) were investigated through a quantitative survey of 18 stations, sampled during a winter period. Factorial correspondence analysis (FCA) of the distribution of main species among stations and automatic hierarchical classification identified seven species groups and five station groups or faunal assemblages, the latter being also characterized in terms of species constancy and fidelity. These assemblages were: (1) a Gastrosaccus spinifer –Nephtys cirrosacommunity on clean, medium sands in the deep entrance channel; (2) a fine –medium sand assemblage characterized by the amphipods Bathyporeia sarsiand Melita palmatain the two main tidal channels, which penetrates deeply in the bay along the hydrological axes of the system; (3) a very localized assemblage of epi- and infauna on a heterogeneous substratum; (4) a modified Abra albacommunity in the muddy, transverse channels; and (5) a facies of the latter dominated by the oligochaete Tubificoides benedeni.Multiple discriminant analysis including seven environmental variables revealed that the most important factors in the ordination of stations were the sediment parameters, especially organic matter and Zosteradebris, followed by the salinity range and water depth. The spatial organization of subtidal infauna in Arcachon Bay appears to be characteristic of a mesotidal coastal lagoon with important water exchanges with the ocean. Average abundance and biomass of macrofauna amounted to 1719 individuals m −2and 6 ·10 gm −2DW, respectively. Density, biomass and species richness were, on average, higher in muddy sands than in clean, sandy bottoms. Comparison of available quantitative data for Arcachon Bay shows that abundance and biomass of macrozoobenthos are lower in subtidal areas than in intertidal habitats.

Tidal flushing of ammonium from intertidal sediments of Ria Formosa, Portugal

Ammonium concentrations were determined in near-bottom water and intertidal surface sediments collected in February and July 1993 at five stations of Ria Formosa, a shallow meso-tidal coastal lagoon in southern Portugal. At each station, samples were taken a few minutes before tidal inundation, and subsequently 2, 10, 15 and 20 minutes thereafter. Ammonium concentration in near-bottom waters increased dramatically in the first 2 minutes followed by a decrease during the 18 minutes of flooding (maximum range 10.3-2.2 μM). The highest levels in the flooding water were concomitant with a decrease of extractable ammonium recorded in the upper sediment layer (2 cm). Laboratory experiments indicated that ammonium is easily extracted from the sediment solids by physical perturbation, as one would expect when tidal water flushes over the intertidal area. This perturbation results in the export of ammonium from the sediment, by pulse mechanisms of short time intervals. On a daily scale this amount is two orders of magnitude higher than transport resulting from molecular diffusion.