Macroalgal bloom dynamics in a highly eutrophic southern California estuary

A 16-mo long monitoring study was carried out in Upper Newport Bay estuary (UNB), Orange County, California, to quantify the macroalgal community of a southern California estuary. Quarterly sampling began December 1996 at 8 stations along the main channel and tidal creeks ranging from the head to the lower end of UNB. At each station, two strata (one at high and one at low elevation) were surveyed. Macroalgal species abundance (% cover and biomass) and algal tissue nitrogen (N) and phosphorus (P) were measured. The algal community changed from sparse macroalgal cover during winter 1996 to larger patches dominated byEnteromorpha intestinalis in spring 1997. The community was characterized by a thick cover of macroalgae comprised ofE. intestinalis andUlva expansa in summer 1997 andU. expansa andCeramium spp. in fall 1997. UNB returned to sparse macroalgal cover by spring 1998. In summer and fall 1997, biomass ofE. intestinalis andCeramium reached over 1,000 g wet wt m⁻² each, andU. expansa biomass exceeded 700 g wet wt m⁻². Tissue N was high inE. intestinalis andU. expansa collected from UNB (≈3% dry wt) and higher inCeramium (≈3.5% dry wt). Tissue P in all three algae ranged from 0.24–0.28% dry wt. Tissue N∶P (molar) ratios inE. intestinalis andU. expansa ranged from 16.4 to 30.0 and inCeramium from 21.8 to 40.1. A field experiment was conducted in whichE. intestinalis was used as a bioassay of N and P availability. Algal tissue was cultured under known conditions and samples were deployed throughout the estuary and left for 24 h. Tissue N of algae from these bags showed a nominal increase in N with proximity to the primary nutrient input to the system, San Diego Creek (p=0.0251; r²=0.200). Our data indicate that UNB is already a highly eutrophic estuary, but macroalgal blooms in UNB may increase if more N is added to the system.     

Variability in drift macroalgal abundance in relation to biotic and abiotic factors in two seagrass dominated estuaries in the Western Gulf of Mexico

We examined the spatial and temporal variability in drift macroalgal abundance in two seagrass dominated estuarine systems on the Texas coast: Redfish Bay (in the Copano-Aransas Estuary) and Lower Laguna Madre. Measurements of benthic macroalgal variability were made in conjunction with a suite of biotic (seagrass biomass, percent cover, blade width and length, shoot density, epiphyte biomass, seagrass blade C:N ratios, and drift macroalgal abundance and composition) and abiotic (inorganic nitrogen and phosphorus concentrations, chlorophylla, total suspended solids, light attenuation, salinity, temperature, total organic carbon and porewater NH₄ ⁺) indicators. All parameters were measured at 30 sites within each estuary semiannually from July 2002 to February 2004. Principal components analysis (PCA) was used to examine relationships between drift macroalgal abundance and biotic and abiotic parameters. In both Redfish Bay and Lower Laguna Madre, drift macroalgal distribution was widespread, and during three of four sampling periods, abundance was equal to abovegro und biomass ofThalassia testudinum, the dominant seagrass. Drift macro algal abundance was highly variable within sites, between sites, and between seasons in both estuaries. No significant differences in drift macroalgal abundance were found between Redfish Bay and Lower Laguna Madre. In Redfish Bay, drift macroalgae (90.1±10.2 gm⁻²) tended to accumulate in bare patches within seagrass beds. In Lower Laguna Madre, drift macroalgae (72.7±10.7 gm⁻²) tended to accumulate in areas of dense seagrass cover rather than in bare areas. We found no relationship between drift macroalgal abundance and low (<2μM) water column nutrient concentrations, and although several of our measured parameters were related to drift macroalgal abundance, none alone sufficiently explained the variability in abundance noted between the two estuarine systems. The contrasting patterns of macroalgal accumulation between Redrish Bay and Lower Laguna Madre likely reflect differences in water circulation characteristics between the two regions as dictated by local physiography, in cluding the shape and orientation of the lagoons, with seasonal variations in macroalgal abundance related to changes in freshwater inflow and nutrient loading.     

Oxygen and nutrient dynamics within mats of the filamentous macroalga Chaetomorpha linum

Concentration profiles of O₂, NH₄ ⁺, NO₃ ⁻, and PO₄ ³⁻ were measured at high spatial resolution in a 12-cm thick benthic mat of the filamentous macroalga Chaetomorpha linum. Oxygen and nutrient concentration profiles varied depending on algal activity and water turbulence. High surface irradiance stimulated O₂ production in the surface layers and introduced O₂ to deeper parts of the mat while the bottom layers of the mat and the underlying sediment were anoxic. Nutrient concentrations were highest in the bottom layers of the mat directly above the sediment nutrient source and decreased towards the surface layers due to algal assimilation and enhanced mixing with the overlying water column. Increased turbulence during windy periods resulted in more homogeneous oxygen and nutrient concentration profiles and shifted the oxic-anoxic interface downward. Denitrification within the mat, as measured by the isotope pairing technique on addition of ¹⁵NO₃ ⁻, was found to take place directly below the oxic-anoxic interface. Denitrification activity was always due to coupled nitrification-denitrification, whereby nitrifiers in the mat utilize NH₄ ⁺ diffusing from below and O₂ diffusing from above. The denitrification rate in the mat ranged from 22 μmol m⁻² h⁻¹ to 28 μmol m⁻² h⁻¹, approximately equivalent to that measured in the surrounding nonvegetated sediment. Although sediment denitrification is suppressed when the sediment surface is covered by a dense macroalgal mat, the denitrification zone may migrate up into the mat. In eutrophic estuaries with a large area of macroalgal cover, the physical structure and growth stage of algal mats may thus play an important role in the regulation of nitrogen removal by denitrification.     

Application of color infrared aerial photography to assess macroalgal distribution in an eutrophic estuary,Upper Newport Bay,California

Newport Bay is a large estuary in southern California that is subject to anthropogenic nutrient loading, eutrophication, and hypoxia. Ground-based methods of assessing algal extent for monitoring and management are limited in that they cannot provide a synoptic view of algal distribution over comparatively large areas. The goal of this study was to explore the application of color infrared aerial photography as an alternative for analyzing the changes in the abundance of exposed macroalgae. Three surveys combining remote sensing (color infrared aerial photography) and ground-based sampling to quantify macroalgal mat coverage were carried out in Upper Newport Bay (UNB) between July and October 2005. Airborne photographs (scale 1:6000) collected during daytime low tides, clear skies, and appropriate sun angle were digitized to 25-cm resolution, orthorectified, georegistered, and combined into three mosaic composite digital images: one for each survey. During each aerial photography survey, macroalgal percent cover was measured on the ground by the pointintercept method in a 6.25-m² area at ca. 30 locations distributed along the water’s edge throughout the intertidal mudflat area. There were three main types of cover:Ulva spp. (green algae),Ceranrium spp. (red algae), and bare surface (mud and mussel beds). To analyze similarities between spectral signatures in the images and cover types, the pixels corresponding to the ground samples from each survey were grouped into clusters based on similarity of their spectral signatures. To establish relationships between spectral signatures in the images and cover as determined from ground data, pixels in each composite image corresponding to ground samples from the same day that were characterized by > 90% of one cover type were attributed to that cover type. Ground samples comprised of a mixture of cover types were used for accuracy assessment. Before classification, each digital image was transformed by the Minimum Noise Fraction Rotation method to remove noise and enhance contrast between the classes. For classification of each composite image, the Spectral Angle Mapper scheme was used: all pixels in each image were attributed to the identified classes and the areal extent of each class was estimated. According to these assessments, the macroalgal coverage in UNB increased from 37% in July to 57% in September to 80% in October, and during this timeUlva spp. replacedCeramium spp. as the dominant alga. This analysis showed that color infrared aerial photography is an effective tool for assessing estuarine, intertidal macroalgal coverage.     

Effects of Torul dam on water quality in the stream Harşit,NE Turkey

This paper evaluates the effects of Torul dam on the stream Harşit water quality in terms of 13 physico-chemical parameters in the Gümüşhane Province, Eastern Black Sea Basin, Turkey. For this purpose, a study was fortnightly conducted during the four seasons between March 2009 and February 2010. In two monitoring stations selected in the upstream and downstream of the Torul dam, T, pH, DO and EC were determined in situ, and collected water samples were analyzed for TH, COD, NH₄ ⁺-N, NO₂ ⁻-N, NO₃ ⁻-N, TN, TKN, PO₄ ³⁻-P and MBAS. According to the Turkish Water Pollution Control Regulation (TWPCR), the stream Harşit was classified, and the obtained results were evaluated for the values proposed by Turkish Standard (TS) 266 and World Health Organization (WHO) guidelines. The results showed that the stream Harşit has high-quality water in terms of, T, pH, DO, COD, NH₄ ⁺-N and NO₃ ⁻-N, but slightly polluted water in terms of NO₂ ⁻-N, TKN and PO₄ ³⁻-P, and polluted for MBAS. It was concluded that Torul dam has a positive effect on the stream water quality in terms of decrease in the annual average concentration values. The percent decreases for TH, COD, NH₄ ⁺-N, NO₂ ⁻-N, NO₃ ⁻-N, TN, TKN, PO₄ ³⁻-P and MBAS were 17.1, 20.3, 56.2, 62.6, 11.7, 11.9, 11.4, 17.8 and 71.4, respectively. The reason for these decreases is probably due to the Torul dam reservoir where the water has a hydraulic residence time and the exposure to chemicals by aquatic organisms or populations that ingest the water. Also, statistical analysis shows that there are significant correlations among the studied parameters.     

Evolution of tidal creek networks in a high sedimentation environment: A 5-year experiment at Tijuana Estuary, California

In a large (8 ha) salt marsh restoration site, we tested the effects of excavating tidal creeks patterned after reference systems. Our purposes were to enhance understanding of tidal creek networks and to test the need to excavate creeks during salt marsh restoration. We compared geomorphic changes in areas with and without creek networks (n = 3; each area 1.3 ha) and monitored creek cross-sectional areas, creek lengths, vertical accretion, and marsh surface elevations for 5 yr that included multiple sedimentation events. We hypothesized that cells with creeks would develop different marsh surface and creek network characteristics (i.e., surface elevation change, sedimentation rate, creek cross-sectional area, length, and drainage density). Marsh surface vertical accretion averaged 1.3 cm yr⁻¹ with large storm inputs, providing the opportunity to assess the response of the drainage network to extreme sedimentation rates. The constructed creeks initially filled due to high accretion rates but stabilized at cross-sectional areas matching, or on a trajectory toward, equilibrium values predicted by regional regression equations. Sedimentation on the marsh surface was greatest in low elevation areas and was not directly influenced by creeks. Time required for cross-sectional area stabilization ranged from 0 to > 5 yr, depending on creek order. First-order constructed creeks lengthened rapidly (mean rate of 1.3 m yr⁻¹) in areas of low elevation and low vegetation cover. New (volunteer) creeks formed rapidly in cells without creeks in areas with low elevation, low vegetation cover, and high elevation gradient (mean rate of 6.2 m yr⁻¹). After 5 yr, volunteer creeks were, at most, one-fourth the area of constructed creeks and had not yet reached the upper marsh plain. In just 4 yr, the site’s drainage density expanded from 0.018 to reference levels of 0.022 m m⁻². Pools also formed on the marsh plain due to sediment resuspension associated with wind-driven waves. We conclude that excavated creeks jump-started the development of drainage density and creek and channel dimensions, and that the tidal prism became similar to those of the reference site in 4–5 yr.     

Denitrification and the stoichiometry of nutrient regeneration in Waquoit Bay, Massachusetts

To determine the removal of regenerated nitrogen by estuarine sediments, we compared sediment N₂ fluxes to the stoichiometry of nutrient and O₂ fluxes in cores collected in the Childs River, Cape Cod, Massachusetts. The difference between the annual PO₄ ³⁻ (0.2 mol P m⁻² yr⁻¹) and NH₄ ⁺ (1.6 mol N m⁻² yr⁻¹) flux and the Redfield N∶P ratio of 16 suggested an annual deficit of 1.5 mol N m⁻² yr⁻¹. Denitrification predicted from O₂∶NH₄ ⁺ flux ratios and measured as N₂ flux suggested a nitrogen sink of roughly the same magnitude (1.4 mol N m⁻² yr⁻¹). Denitrification accounted for low N∶P ratios of benthic flux and removed 32–37% of nitrogen inputs entering the relatively highly nutrient loaded Childs River, despite a relatively brief residence time for freshwater in this system. Uptake of bottom water nitrate could only supply a fraction of the observed N₂ flux. Removal of regenerated nitrogen by denitrification in this system appears to vary seasonally. Denitrification efficiency was inversely correlated with oxygen and ammonium flux and was lowest in summer. We investigated the effect of organic matter on denitrification by simulating phytoplankton deposition to cores incubated in the lab and by deploying chambers on bare and macroaglae covered sediments in the field. Organic matter addition to sediments increased N₂ flux and did not alter denitrification efficiency. Increased N₂ flux co-varied with O₂ and NH₄ ⁺ fluxes. N₂ flux (261±60 μmol m⁻² h⁻¹) was lower in chambers deployed on macroalgal beds than deployed on bare sediments (458±70 μmol m⁻² h⁻¹), and O₂ uptake rate was higher in chambers deployed on macroalgal beds (14.6±2.2 mmol m⁻² h⁻¹) than on bare sediments (9.6±1.5 mmol m⁻² h⁻¹). Macroalgal cover, which can retain nitrogen in the system, is a link between nutrient loading and denitrification. Decreased denitrification due to increasing macroalgal cover could create a positive feedback because decreasing denitrification would increase nitrogen availability and could increase macroalgae cover.     

Variations and net transport of dissolved inorganic nutrients in the South Passage of the Changjiang （Yangtze River） Estuary

Dissolved inorganic nutrient elements were analyzed from the samples collected in the South Passage of the Changjiang (Yangtze River) Estuary in March 2003, including NH4 , NO3-, NO2- and PO43-. The water samples were collected with a Niskin sampler hourly at the near-surface, middle and near-bottom depths at the three stations -A1, A2 and A3-during two complete tidal cycles of neap tide and spring tide. Results showed that 1) the concentrations of NH4 , NO3- and NO2- were a little higher respectively during the neap tide than those during the spring tide, while PO43- showed an opposite trend, and each was higher in the ebb tide than in the flood tide, either for the neap tidal cycle or the spring tidal cycle; 2) higher stratification of the nutrients existed obviously in this area, with the concentrations of which increased from the bottom to the surface, especially for NH4 and NO3-; 3) the coefficient of variation (C.V.) values of all dissolved inorganic nutrients varied from 4.06% to 36.8% beyond different influences of the tidal current and Changjiang runoff; 4) with increasing suspended matter in the water column, the concentrations of PO43- became lower in the filtered water; and 5) the total transport of each tidal cycle was much more in the spring tide than in the neap tide, and the positive values indicated that the nutrients had been exported to the East China Sea. Studies on the variations and net transport of dissolved inorganic nutrients in the South Passage of the Changjiang Estuary will provide the scientific basis for the study of the mechanism of red tide in the East China Sea.     

Tidally-induced shear stress variability above intertidal mudflats in the macrotidal seine estuary

Tidal currents and the spatial variability of tidally-induced shear stress were studied during a tidal cycle on four intertidal mudflats from the fluvial to the marine part of the Seine estuary. Measurements were carried out during low water discharge (<400 m³ s⁻¹) in neap and spring tide conditions. Turbulent kinetic energy, covariance, and logarithmic profile methods were used and compared for the determination of shear stress. The c_TKE coefficient value of 0.19 cited in the literature was confirmed. Shear stress values were shown to decrease above mudflats from the mouth to the fluvial part of the estuary due to dissipation of the tidal energy, from 1 to 0.2 N m⁻² for spring tides and 0.8 to 0.05 N m⁻² for neap tides. Flood currents dominate tidally-induced shear stress in the marine and lower fluvial estuary during neap and spring tides and in the upper fluvial part during spring tides. Ebb currents control tidally-induced shear stress in the upper fluvial part of the estuary during neap tides. These results revealed a linear relationship between friction velocities and current velocities. Bed roughness length values were calculated from the empirical relationship given by Mitchener and Torfs (1996) for each site; these values are in agreement with the modes of the sediment particle-size distribution. The influence of tidal currents on the mudflat dynamics of the Seine estuary was examined by comparing the tidally-induced bed shear stress and the critical erosion shear stress estimated from bed sediment properties. Bed sediment resuspension induced by tidal currents was shown to occur only in the lower part of the estuary.     

Macrofaunal Spatial Patterns in Relationship to Environmental Variables in the Richibucto Estuary,New Brunswick,Canada

Estuarine macrobenthos respond to a variety of environmental gradients such as sediment type and salinity, and organic enrichment. A relatively new influence, organic loading from suspended bivalve culture, has the potential to alter this response. A study on soft-bottom macrobenthic communities was carried out in the Richibucto estuary (46°40′N, 64°50′W), New Brunswick, Canada, with samples collected from 18 stations in late September and early October 2006. The site consisted of a large tidal channel originating upstream in a small river. The channel was punctuated by bag culture of oysters along its length. A total of 88 species were recorded. The mean values of abundance, species richness, and diversity (H′) of macrofauna were 11,199 ind. m⁻² (ranged from 4,371 to 19,930 ind. m⁻²), 23.4 species grab⁻¹ and 3.29 grab⁻¹, respectively. In general species richness and H′ increased from the upper estuary to the estuarine mouth. Multivariate analyses clearly exhibited the spatial distribution in community structure, which coincided with the locations along the estuary (the upper, the lower and the mouth), as well as inside and outside the channel. Species richness and diversity H′ showed strong positive correlations with salinity (21.2–25.2 ppt), and abundance was positively correlated with water depth (1.0–4.5 m). Abundance and species richness were negatively correlated with both of silt–clay fraction (3.3–24.8%) and sorting (σ_I). Species richness was also negatively correlated with organic content (1.9–12.7%). The BIO-ENV analyses identified silt–clay fraction, σ_I and salinity as the major environmental variable combination influencing the macrofaunal patterns, and silt–clay fraction as the single best-correlated variable.     

Net aboveground primary production and soil properties of floating and attached freshwater tidal marshes in the Río de la Plata estuary,Argentina

In the lower delta of the Paraná River, at the head of the Río de la Plata estuary (Argentina), we compared net aboveground primary production (NAPP) and soil properties of the dominant macrophyteScirpus giganteus (Kunth) in a floating and an attached marsh community. Both marshes are tidally influenced but in different ways. The floating marsh site is relatively isolated from tidal influences because its ability to float makes it resistant to overland flow and to sediment inputs from the estuary. The attached marsh lacks the capacity to float and receives sediment supplies from the estuary through overland flow. These hydrologic differences are reflected in lower mineral content in sediments of the floating marsh. Using a leaf tagging technique, estimated NAPP was 1,109 ± 206 g m⁻² yr⁻¹ for the floating marsh and 1,866 ±258 g m⁻² yr⁻¹ for the attached marsh. We attribute the lower NAPP of the floating marsh to isolation from sediment input from overland flow.     

The Contribution of Benthic Nutrient Regeneration to Primary Production in a Shallow Eutrophic Estuary,Weeks Bay,Alabama

Benthic oxygen, dinitrogen, and nutrient fluxes (NH₄⁺, NO₃⁻, and PO₄³⁻) were measured monthly during a 1-year period at two locations in Weeks Bay, a shallow (1.4 m) and eutrophic estuary in Alabama. Gross primary productivity (GPP), ecosystem respiration (R), and net ecosystem metabolism were determined from high-frequency dissolved oxygen measurements. Peak water column NO₃⁻ (55 μM) and chlorophyll a (138 μg/l) concentrations were measured during spring and fall, respectively. Sediments were a net source of NH₄⁺ (102 μmol m⁻² h⁻¹) and PO₄³⁻ (0.9 μmol m⁻² h⁻¹) but a sink for NO₃⁻ (−30 μmol m⁻² h⁻¹). Benthic N₂ fluxes indicated net N fixation (12 μmol N m⁻² h⁻¹). Sediment oxygen demand (0.55 g O₂ m⁻² day⁻¹) accounted for <10% of R (7.3 g O₂ m⁻² day⁻¹). Despite high GPP rates (4.7 g O₂ m⁻² day⁻¹), the estuary was net heterotrophic. Benthic regeneration supplied, on average, 7.5% and 4% of primary productivity N and P demands, respectively. These results contrast with the conventional view that benthic regeneration accounts for a large fraction of phytoplankton nutrient demand in shallow estuaries.     

Benthic nutrient fluxes in the intertidal flat within the Changjiang （Yangtze River） Estuary

In an annual cycle from March 2005 to February 2006, benthic nutrient fluxes were measured monthly in the Dongtan intertidal flat within the Changjiang （Yangtze River） Estuary. Except for NH4^＋, there always showed high fluxes from overlying water into sediment for other four nutrients. Sediments in the high and middle marshes, covered with halophyte and consisting of macrofauna, demonstrated more capabilities of assimilating nutrients from overlying water than the low marsh. Sampling seasons and nutrient concentrations in the overlying water could both exert significant effects on these fluxes. Additionally, according to the model provided by previous study, denitrification rates, that utilizing NO3- transported from overlying water （Dw） in Dongtan sediments, were estimated to be from -16 to 193 μmol·h^-1·m^-2 with an average value of 63 μmol·h^-1·m^-2 （n=18）. These estimated values are still underestimates of the in-situ rates owing to the lack of consideration of DN, i.e., denitrification supported by the local NO3^- production via nitrification.     

Effects of Environmental Variation and Non-point Source (NPS) Nutrient Pollution on Aquatic Plant Communities in Weeks Bay National Estuarine Research Reserve (WBNERR), AL

This study was designed to investigate non-point source nutrient pollution and its influences on submerged aquatic plant community structure and biological invasion in the Weeks Bay National Estuarine Research Reserve (WBNERR). A monthly vegetation survey was conducted to document plant abundance and changes in community structure; physicochemical data and water samples were collected on a bi-monthly basis to monitor environmental conditions (i.e., pH, salinity, dissolved oxygen, light intensity, and attenuation) and water column nutrient (NO₂⁻/NO₃⁻ and NH₄⁺) and chlorophyll a concentrations. A total of seven submerged aquatic species were identified at the WBNERR with the occurrence of only one non-native species (Hydrilla verticillata). Statistical analyses suggest that water column nitrogen concentrations along with variations in dissolved oxygen (0.6–11.5 mg L⁻¹), light attenuation, pH (5.6–8.6), and temperature (11–33°C) play key roles in determining the aquatic plant abundance and distribution in the WBNERR.     

Benthic metabolism and nutrient cycling along an estuarine salinity gradient

Benthic metabolism and nutrient exchange across the sediment-water interface were examined over an annual cycle at four sites along a freshwater to marine transect in the Parker River-Plum Island Sound estuary in northeastern Massachusetts, U.S. Sediment organic carbon content was highest at the freshwater site (10.3%) and decreased along the salinity gradient to 0.2% in the sandy sediments at the marine end of the estuary. C:N ratios were highest in the mid estuary (23:1) and lowest near the sea (11:1). Chlorophyll a in the surface sediments was high along the entire length of the estuary (39–57 mg chlorophyll a m⁻²) but especially so in the sandy marine sediments (172 mg chlorophyll a m⁻²). Chlorophyll a to phaeophytin ratios suggested most chlorophyll is detrital, except at the sandy marine site. Porewater sulfide values varied seasonally and between sites, reflecting both changes in sulfate availability as overlying water salinity changed and sediment metabolism. Patterns of sediment redox potential followed those of sulfide. Porewater profiles of inorganic N and P reflected strong seasonal patterns in remineralization, accumulation, and release. Highest porewater NH₄ ⁺ values were found in upper and mid estuarine sediments, occasionally exceeding 1 mM N. Porewater nitrate was frequently absent, except in the sandy marine sediments where concentrations of 8 μM were often observed. Annual average respiration was lowest at the marine site (13 mmol O₂ m⁻² d⁻¹ and 21 mmol TCO₂ m⁻² d⁻¹) and highest in the mid estuary (130 mmol O₂ m⁻² d⁻¹ and 170 mmol TCO₂ m⁻² d⁻¹) where clam densities were also high. N₂O and CH₄ fluxes were low at all stations throughout the year: Over the course, of a year, sediments varied from being sources to sinks of dissolved organic C and N, with the overall spatial pattern related closely to sediment organic content. There was little correlation between PO₄ ³⁻ flux and metabolism, which we attribute to geochemical processes. At the two sites having the lowest salinities, PO₄ ³⁻ flux was directed into the sediments. On average, between 22% and 32% of total system metabolism was attributable to the benthos. The mid estuary site was an exception, as benthic metabolism accounted for 95% of the total, which is attributable to high densities of filter-feeding clams. Benthic remineralization supplied from less than 1% to over 190% of the N requirements and 0% to 21% of the P requirements of primary producers in this system. Estimates of denitrification calculated from stoichiometry of C and N fluxes ranged from 0% for the upper and mid estuary site to 35% for the freshwater site to 100% of sediment organic N remineralization at the marine site. We hypothesize that low values in the upper and mid estuary are attributable to enhanced NH₄ ⁺ fluxes during summer due to desorption of exchangeable ammonium from rising porewater salinity. NH₄ ⁺ desorption during summer may be a mechanism that maintains high rates of pelagic primary production at a time of low inorganic N inputs from the watershed.     

Macrophyte Abundance in Waquoit Bay: Effects of Land-Derived Nitrogen Loads on Seasonal and Multi-Year Biomass Patterns

Anthropogenic inputs of nutrients to coastal waters have rapidly restructured coastal ecosystems. To examine the response of macrophyte communities to land-derived nitrogen loading, we measured macrophyte biomass monthly for 6 years in three estuaries subject to different nitrogen loads owing to different land uses on the watersheds. The set of estuaries sampled had nitrogen loads over the broad range of 12 to 601 kg N ha⁻¹ year⁻¹. Macrophyte biomass increased as nitrogen loads increased, but the response of individual taxa varied. Specifically, biomass of Cladophora vagabunda and Gracilaria tikvahiae increased significantly as nitrogen loads increased. The biomass of other macroalgal taxa tended to decrease with increasing load, and the relative proportion of these taxa to total macrophyte biomass also decreased. The seagrass, Zostera marina, disappeared from the higher loaded estuaries but remained abundant in the estuary with the lowest load. Seasonal changes in macroalgal standing stock were also affected by nitrogen load, with larger fluctuations in biomass across the year and higher minimum biomass of macroalgae in the higher loaded estuaries. There were no significant changes in macrophyte biomass over the 6 years of this study, but there was a slight trend of increasing macroalgal biomass in the latter years. Macroalgal biomass was not related to irradiance or temperature, but Z. marina biomass was highest during the summer months when light and temperatures peak. Irradiance might, however, be a secondary limiting factor controlling macroalgal biomass in the higher loaded estuaries by restricting the depth of the macroalgal canopy. The relationship between the bloom-forming macroalgal species, C. vagabunda and G. tikvahiae, and nitrogen loads suggested a strong connection between development on watersheds and macroalgal blooms and loss of seagrasses. The influence of watershed land uses largely overwhelmed seasonal and inter-annual differences in standing stock of macrophytes in these temperate estuaries.     

Context-Dependent Eelgrass–Macroalgae Interactions Along an Estuarine Gradient in the Pacific Northwest,USA

Land-based eutrophication is often associated with blooms of green macroalgae, resulting in negative impacts on seagrasses. The generality of this interaction has not been studied in upwelling-influenced estuaries where oceanic nutrients dominate seasonally. We conducted an observational and experimental study with Zostera marina L. and ulvoid macroalgae across an estuarine gradient in Coos Bay, Oregon. We found a gradient in mean summer macroalgal biomass from 56.1 g dw 0.25 m⁻² at the marine site to 0.3 g dw 0.25 m⁻² at the riverine site. Despite large macroalgal blooms at the marine site, eelgrass biomass exhibited no seasonal or interannual declines. Through experimental manipulations, we found that pulsed additions of macroalgae biomass (+4,000 mL) did not affect eelgrass in marine areas, but it had negative effects in riverine areas. In upwelling-influenced estuaries, the negative effects of macroalgal blooms are context dependent, affecting the management of seagrass habitats subject to nutrient inputs from both land and sea.     

Transport and fate of nitrate within soil units of glacial origin

Questions concerning the influence of soil type and crop cover on the fate and transport of nitrate (NO₃⁻) were examined. During a growing season, soils derived from glacial material underlying either corn or soybeans were sampled for levels of NO₃⁻ within the pore water. Measured levels of NO₃⁻ ranged from below detection limit to 14.9 g NO₃⁻ per kilogram of soil (g/kg). In fields with the same crop cover, the silty-clayey soil exhibited a greater decrease in NO₃⁻ levels with depth than the sandier soil. Crop uptake of NO₃⁻ occurs within the root zone; however, the type of crop cover did not have a direct impact on the fate or transport during the growing season. The soils underlying soybeans had an increase in NO₃⁻ levels following harvest, suggesting that the decomposition of the soybean roots contributed to the net gain of NO₃⁻ in the shallow soil. For all of the soil types, conditions below 100 cm are conducive for microbial denitrification, with both a high water saturation level (>60%) and moderate organic carbon content (1–2%). At depths below 100 cm, temporal differences in NO₃⁻ levels of over a magnitude, up to a 95% reduction, were recorded in the soil units as the growing season progressed. Physical properties that control the transport of NO₃⁻ or denitrification have a larger influence on NO₃⁻ levels than crop type.

Nutrient transport in a riverine-influenced,tidal freshwater bayou in Louisiana

Transport of ammonium (NH₄ ⁺), nitrate + nitrite (NO₃ ^?), total Kjeldahl nitrogen (TKN), soluble reactive phosphate (SRP), and total suspended solids (TSS) was measured in a freshwater tidal bayou located in a marsh system near the mouth of the Atchafalaya River in Louisiana. Sampling was conducted six times over one year and was timed to assess effects of seasonal variation in river flow and mean sea level of the Gulf of Mexico on material fluxes. Net fluxes of all materials were large and ebb directed in all seasons except fall, when net transport was 2 to 3 orders-of-magnitude smaller than in any other season. These results demonstrate that riverine forcing was the primary influence on materials transport in all seasons except fall when tidal forcing was most important. The range of net fluxes (g s^?1) for each nutrient was as follows (a negative sign indicates a net export toward the Gulf): NO₃ ^?, ?0.006 to ?6.69; TKN, 0.09 to ?10.41; NH₄ ⁺, ?0.02 to ?1.36; SRP, ?0.001 to ?0.53; TSS, ?2 to ?81. Analysis of nutrient concentrations indicated the marsh/aquatic system removed NO₃ ^?, SRP, and TSS from the water column from late spring through early fall and released NH₄ ⁺ and TKN in summer. The results of this study show that net materials export per unit cross section channel area increased as riverine influence increased.     

Nutrient limitation of the macroalgaEnteromorpha intestinalis collected along a resource gradient in a highly eutrophic estuary

We conducted a laboratory experiment to quantify nutrient (nitrogen and phosphorus) limitation of macroalgae collected along a gradient in water column nutrient availability in Upper Newport Bay estuary, a relatively nutrient-rich system in southern California, United States. We collectedEnteromorpha intestinalis and water for use in the experiment from five sites ranging from the lower end of the estuary to the head. Initial algal tissue N and P concentrations and molar N∶P ratios—as well as water column NO₃ and total Kjeldahl nitrogen (TKN)—increased along a spatial gradient from the lower end toward the head. Water column soluble reactive phosphorus (SRP) varied among sites as well but did not follow a pattem of increasing from the seaward end toward the head. Algae from each site were assigned to one of four experimental treatments: control (C), nitrogen enrichment (+N), phosphorus enrichment (+P), and nitrogen and phosphorus enrichment (+N+P). Each week for 3 wk we replaced the water in each unit with the appropriate treatment water to mimic a poorly flushed estuary. After 3 wk, the degree of nutrient limitation ofE. intestinalis varied spatially with distance from the head of the estuary. Growth ofE. intestinalis collected from several sites increased with N enrichment alone and increased further when P was added in combination with N This indicated that N was limiting and that when N was sufficient, P became limiting. Sites from whichE. intestinalis exhibited nutrient limitation spanned the range of background water column NO₃ (12.9±0.4 to 55.2±2.1 μM) and SRP (0.8±0.0 to 2.9±0.2 μM) concentrations. Algae that were N limited had initial tissue N levels ranging from 1.18±0.03 to 2.81±0.08% dry weight and molar N∶P ratios ranging from 16.75±0.39 to 26.40±1.98.