Seagrasses, marine flowering plants, have a long evolutionary history but are now challenged with rapid environmental changes as a result of coastal human population pressures. Seagrasses provide key ecological services, including organic carbon production and export, nutrient cycling, sediment stabilization, enhanced biodiversity, and trophic transfers to adjacent habitats in tropical and temperate regions. They also serve as “coastal canaries,” global biological sentinels of increasing anthropogenic influences in coastal ecosystems, with large-scale losses reported worldwide. Multiple stressors, including sediment and nutrient runoff, physical disturbance, invasive species, disease, commercial fishing practices, aquaculture, overgrazing, algal blooms, and global warming, cause seagrass declines at scales of square meters to hundreds of square kilometers. Reported seagrass losses have led to increased awareness of the need for seagrass protection, monitoring, management, and restoration. However, seagrass science, which has rapidly grown, is disconnected from public awareness of seagrasses, which has lagged behind awareness of other coastal ecosystems. There is a critical need for a targeted global conservation effort that includes a reduction of watershed nutrient and sediment inputs to seagrass habitats and a targeted educational program informing regulators and the public of the value of seagrass meadows.

https://www.jstor.org/stable/10.1641/0006-3568(2006)56%5B987:AGCFSE%5D2.0.CO;2

A Global Crisis for Seagrass Ecosystems

For open ocean and coastal waters, a multiband quasi-analytical algorithm is developed to retrieve absorption and backscattering coefficients, as well as absorption coefficients of phytoplankton pigments and gelbstoff. This algorithm is based on remote-sensing reflectance models derived from the radiative transfer equation, and values of total absorption and backscattering coefficients are analytically calculated from values of remote-sensing reflectance. In the calculation of total absorption coefficient, no spectral models for pigment and gelbstoff absorption coefficients are used. Actually those absorption coefficients are spectrally decomposed from the derived total absorption coefficient in a separate calculation. The algorithm is easy to understand and simple to implement. It can be applied to data from past and current satellite sensors, as well as to data from hyperspectral sensors. There are only limited empirical relationships involved in the algorithm, and they are for less important properties, which implies that the concept and details of the algorithm could be applied to many data for oceanic observations. The algorithm is applied to simulated data and field data, both non-case1, to test its performance, and the results are quite promising. More independent tests with field-measured data are desired to validate and improve this algorithm.

/pdf/deriving-inherent-optical-properties-from-water-color-a-4gmos8ca0n.pdf

Deriving inherent optical properties from water color: a multiband quasi-analytical algorithm for optically deep waters

Publisher Summary This chapter focuses on the uses of isotopes to understand water chemistry.I Isotopic compositions generally cannot be interpreted successfully in the absence of other chemical and hydrologic data. The chapter focusses on uses of isotopes in tracing sources and cycling of nitrogen in the water-component of forested catchment, and on dissolved nitrate in shallow waters, nutrient uptake studies in agricultural areas, large-scale tracer experiments, groundwater contamination studies, food-web investigations, and uses of compound-specific stable isotope techniques. Shallow waters moving along a flowpath through a relatively uniform material and reacting with minerals probably do not achieve equilibrium but gradually approach some steady-state composition. The chapter also discusses the use of isotopic techniques to assess impacts of changes in land-management practices and land use on water quality. The analysis of individual molecular components for isotopic composition has much potential as a method for tracing the source, biogeochemistry, and degradation of organic liquids and gases because different materials have characteristic isotope spectrums or biomarkers.

Tracing nitrogen sources and cycling in catchments

Stable nitrogen isotope signatures (δ15N) are increasingly used to infer the trophic position of consumers in food web studies. Interpreting the δ15N of consumers relative to the δ15N characterizing the base of the food web provides a time-integrated measure of trophic position. We use primary consumers (trophic level 2) as baseline indicator organisms and investigate the variation in baseline δ15N values in 14 lakes in Ontario and Quebec. Values of δ15N ranged from −2 to +9‰ and varied significantly as a function of lake habitat (mean littoral = 1.6‰, pelagic = 3.1‰, profundal = 5.2 ‰). Stable carbon isotopic signatures (δ13C) of primary consumers decreased along this same habitat gradient (mean littoral = −23.8‰, pelagic = −28.4‰, profundal = −30.5‰). Primary consumer δ13C and a categorical lake variable explained 72% of the variability in primary consumer δ15N. This relationship was corroborated by primary consumer δ15N and δ13C data from the literature, indicating that habitat-specific variation in ba...

PRIMARY CONSUMER δ13C AND δ15N AND THE TROPHIC POSITION OF AQUATIC CONSUMERS

Antimony in the environment: a review focused on natural waters: I. Occurrence

We used a field experiment to assess the individual and combined effects of removing top predators and enriching water column nutrients (nitrogen-N and phosphorus-P) on seagrass ecosystem structure and function. Experiments were conducted in turtlegrass (Thalassia testudinum) habitats in St. Joseph Bay, FL, an aquatic preserve in the northern Gulf of Mexico that exhibits low ambient nutrient concentrations and contains abundant populations of small crustacean and gastropod mesograzers. We stocked 7.0 m 2 enclosures with elevated (;4‐83 ambient) densities of juvenile pinfish (Lagodon rhomboides), the dominant fish species in local seagrass habitats, to simulate the first-order effects of large predator reductions, and we used an in situ delivery system to supplement N and P to ;33 ambient levels in nutrient addition treatments. Monthly determinations of water column nutrients and Chl a, along with measurements of the biomass and abundance of leaf epiphytes and seagrass production, biomass, and shoot and leaf densities were used to evaluate the relative effects of manipulating nutrient supply and altering food web structure. In contrast to our expectations, results showed few significant nutrient effects, or fish 3 nutrient enrichment effects on any of the parameters measured. However, there were many significant fish effects, most of which were unexpected. As predicted, increased pinfish density reduced mesograzer numbers significantly. Not anticipated, however, was the reduced epiphyte biomass in fish enclosure treatments, apparently brought about by the pinfish consuming significant amounts of epiphytes as well as mesograzers. This reduction in epiphyte biomass produced positive indirect effects on seagrass biomass, shoot number, and rates of primary productivity in pinfish enclosure treatments. Our results also showed important top-down effects in determining the composition and abundance of seagrassassociated plants and animals in this pristine environment. Although we did not observe simple trophic cascades, most likely because pinfish fed at more than one trophic level, and because the dense seagrass prevented small grazers from being reduced to low numbers, pinfish produced important changes in the epibiota as well as the seagrasses themselves. These data, while contrasting with studies reporting significant negative nutrient enrichment effects on seagrasses, support the results of recent experimental studies in showing that: (1) small grazers can often control the abundance of epiphytes; and (2) it is unlikely that a full understanding of the consequences of nutrient enrichment for seagrass ecosystems can be gained without knowing how grazer population are regulated.

https://aslopubs.onlinelibrary.wiley.com/doi/pdf/10.4319/lo.2000.45.5.1041

Effects of nutrient enrichment and small predator density on seagrass ecosystems: An experimental assessment

Isotopic fractionation of ammonium (NH,+ ) and nitrate (NO,-) during uptake by phytoplankton was examined in batch culture experiments with the diatom SkeZetonema costatum under nitrogen-enriched conditions (5-100 PM). The fractionation factor (E) for NOJ- uptake by Skeletonema was -9.Of0.7%0 and was concentration-independent. For NH,+, E was more variable and dependent on ambient NH,+ concentration. For NH4+ concentration ranges of 100-50, 50-20, and 20-5 PM, E was -24.6k5.5, -27.2-+ 1.6, and -7.8+3.0%. In these cultures, isotopic fractionation by phytoplankton caused variations in 615N of up to 50?& for NH4+, 1 ~%XI for NO,-, and 25% for particulate N. Similar variability in the 615N of both dissolved inorganic and particulate organic N pools should be expected during phytoplankton blooms in nature. As a result, phytoplankton-mediated isotopic variability must be considered when isotopic data are used to examine biogeochemical and physical processing of organic matter in marine ecosystems, particularly when biosynthesis and loss processes are decoupled in either space or time during bloom conditions.

https://aslopubs.onlinelibrary.wiley.com/doi/pdf/10.4319/lo.1996.41.3.0451

Isotopic fractionation of ammonium and nitrate during uptake by Skeletonema costatum: Implications for δ15N dynamics under bloom conditions

Phytoplankton chlorophyll concentrations in the Delaware estuary range over two orders of magnitude and display several maxima over the seasonal cycle. These maxima were found to be regulated both spatially and temporally by light availability. Both the spring chlorophyll maximum, which reaches 50–60 μg chlorophyll l−1 during a Skeletonema costatum dominated bloom, and transient fall blooms (15–20 μg l−1) are focused in mid-estuary. These blooms are regulated spatially by settling out of suspended sediment below the turbidity maximum and both spatially and temporally by physical factors (e.g. river flow) that cause vertical stratification in mid-estuary. In freshwater regions, chlorophyll concentrations display seasonal periodicity correlated with solar irradiance; summer chlorophyll concentrations average 30 μg l−1. These freshwater and mid-estuarine biomass maxima may be correctly predicted using a steady-state light-limitation model. In contrast, summer chlorophyll concentrations in the lower estuary remain below 10 μg l−1 and are not correctly modeled, despite minimum turbidity, and non-nutrient limiting conditions. These chlorophyll concentrations appear to be regulated by a combination of light availability and grazing.

Although extremely high anthropogenic nutrient inputs in the freshwater region of the Delaware River provide non-limiting nutrient concentrations throughout the estuary, regulation of phytoplankton growth by light-limitation restricts chlorophyll concentrations below the nuisance levels found in many eutrophic systems.

Jonathan Pennock

Papers

Effects of nutrient enrichment and small predator density on seagrass ecosystems: An experimental assessment

Isotopic fractionation of ammonium and nitrate during uptake by Skeletonema costatum: Implications for δ15N dynamics under bloom conditions

Chlorophyll distributions in the Delaware estuary: Regulation by light-limitation

Biogeochemical factors that influence the stable nitrogen isotope ratio of dissolved ammonium in the Delaware Estuary

Biogeochemistry of Gulf of Mexico Estuaries