William C. Dennison

Accelerating loss of seagrasses across the globe threatens coastal ecosystems

Coastal ecosystems and the services they provide are adversely affected by a wide variety of human activities. In particular, seagrass meadows are negatively affected by impacts accruing from the billion or more people who live within 50 km of them. Seagrass meadows provide important ecosystem services, including an estimated

A Global Crisis for Seagrass Ecosystems

1.9 trillion per year in the form of nutrient cycling; an order of magnitude enhancement of coral reef fish productivity; a habitat for thousands of fish, bird, and invertebrate species; and a major food source for endangered dugong, manatee, and green turtle. Although individual impacts from coastal development, degraded water quality, and climate change have been documented, there has been no quantitative global assessment of seagrass loss until now. Our comprehensive global assessment of 215 studies found that seagrasses have been disappearing at a rate of 110 km2 yr−1 since 1980 and that 29% of the known areal extent has disappeared since seagrass areas were initially recorded in 1879. Furthermore, rates of decline have accelerated from a median of 0.9% yr−1 before 1940 to 7% yr−1 since 1990. Seagrass loss rates are comparable to those reported for mangroves, coral reefs, and tropical rainforests and place seagrass meadows among the most threatened ecosystems on earth.

Assessing Water Quality with Submersed Aquatic Vegetation

ABSTRACT 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.

Effects of light on seagrass photosynthesis, growth and depth distribution☆

Estuaries throughout the world are experiencing water quality problems as the result of human population growth in coastal areas. By establishing the habitat requirements of critical submerged aquatic vegetation, water quality can be evaluated and restoration goals can be made. This study used submerged vegetation in Chesapeake Bay to examine the habitat and health of the Bay. Both natural distributions and transplant survival in different studies were analyzed. The five habitat requirements used were light attenuation, total suspended solids, chlorophyll, dissolved inorganic nitrogen, and dissolved inorganic phosphorus. Water-quality conditions supporting vegetation growth to one meter depth was used. This study represents the first attempt at linking habitat requirements of a living resource to water quality standards in an estuarine system. It allows for predictive capability without detailed knowledge of the precise nature of vegetation/water quality interactions.

A new approach for detecting and mapping sewage impacts

Abstract The relationships between light regime, photosynthesis, growth and depth distribution of a temperate seagrass, Zostera marina L. (eelgrass), were investigated in a subtidal eelgrass meadow near Woods Hole, MA. The seasonal light patterns in which the quantum irradiance exceeded the light compensation point ( H comp ) and light saturation point ( H sat ) for eelgrass photosynthesis were determined. Along with photosynthesis and respiration rates, these patterns were used to predict carbon balances monthly throughout the year. Gross photosynthesis peaked in late-summer, but net photosynthesis peaked in spring (May), due to high respiration rates at summer temperatures. Predictions of net photosynthesis correlated with in situ growth rates at the study site and with reports from other locations. The maximum depth limit for eelgrass was related to the depth distribution of H comp , and a minimum annual average H comp (12.3 h) for survival was determined. Maximum depth limits for eelgrass were predicted for various light extinction coefficients and a relationship between Secchi disc depth and the maximum depth limit for survival was established. The Secchi disc depth averaged over the year approximates the light compensation depth for eelgrass. This relationship may be applicable to other sites and other seagrass species.

ENCORE: The effect of nutrient enrichment on coral reefs. Synthesis of results and conclusions

Increased nitrogen loading has been implicated in eutrophication occurrences worldwide. Much of this loading is attributable to the growing human population along the worlds coastlines. A significant component of this nitrogen input is from sewage effluent, and delineation of the distribution and biological impact of sewage-derived nitrogen is becoming increasingly important. Here, we show a technique that identifies the source, extent and fate of biologically available sewage nitrogen in coastal marine ecosystems. This method is based on the uptake of sewage nitrogen by marine plants and subsequent analysis of the sewage signature (elevated delta 15N) in plant tissues. Spatial analysis is used to create maps of delta 15N and establish coefficient of variation estimates of the mapped values. We show elevated delta 15N levels in marine plants near sewage outfalls in Moreton Bay, Australia, a semi-enclosed bay receiving multiple sewage inputs. These maps of sewage nitrogen distribution are being used to direct nutrient reduction strategies in the region and will assist in monitoring the effectiveness of environmental protection measures.

Photosynthetic responses of Zostera marina L. (Eelgrass) to in situ manipulations of light intensity

Coral reef degradation resulting from nutrient enrichment of coastal waters is of increasing global concern. Although effects of nutrients on coral reef organisms have been demonstrated in the laboratory, there is little direct evidence of nutrient effects on coral reef biota in situ. The ENCORE experiment investigated responses of coral reef organisms and processes to controlled additions of dissolved inorganic nitrogen (N) and/or phosphorus (P) on an offshore reef (One Tree Island) at the southern end of the Great Barrier Reef, Australia. A multi-disciplinary team assessed a variety of factors focusing on nutrient dynamics and biotic responses. A controlled and replicated experiment was conducted over two years using twelve small patch reefs ponded at low tide by a coral rim. Treatments included three control reefs (no nutrient addition) and three + N reefs (NH4Cl added), three + P reefs (KH2PO4 added), and three + N + P reefs. Nutrients were added as pulses at each low tide (ca twice per day) by remotely operated units. There were two phases of nutrient additions. During the initial, low-loading phase of the experiment nutrient pulses (mean dose = 11.5 microM NH4+; 2.3 microM PO4(-3)) rapidly declined, reaching near-background levels (mean = 0.9 microM NH4+; 0.5 microM PO4(-3)) within 2-3 h. A variety of biotic processes, assessed over a year during this initial nutrient loading phase, were not significantly affected, with the exception of coral reproduction, which was affected in all nutrient treatments. In Acropora longicyathus and A. aspera, fewer successfully developed embryos were formed, and in A. longicyathus fertilization rates and lipid levels decreased. In the second, high-loading, phase of ENCORE an increased nutrient dosage (mean dose = 36.2 microM NH4+; 5.1 microM PO4(-3)) declining to means of 11.3 microM NH4+ and 2.4 microM PO4(-3) at the end of low tide) was used for a further year, and a variety of significant biotic responses occurred. Encrusting algae incorporated virtually none of the added nutrients. Organisms containing endosymbiotic zooxanthellae (corals and giant clams) assimilated dissolved nutrients rapidly and were responsive to added nutrients. Coral mortality, not detected during the initial low-loading phase, became evident with increased nutrient dosage, particularly in Pocillopora damicornis. Nitrogen additions stunted coral growth, and phosphorus additions had a variable effect. Coral calcification rate and linear extension increased in the presence of added phosphorus but skeletal density was reduced, making corals more susceptible to breakage. Settlement of all coral larvae was reduced in nitrogen treatments, yet settlement of larvae from brooded species was enhanced in phosphorus treatments. Recruitment of stomatopods, benthic crustaceans living in coral rubble, was reduced in nitrogen and nitrogen plus phosphorus treatments. Grazing rates and reproductive effort of various fish species were not affected by the nutrient treatments. Microbial nitrogen transformations in sediments were responsive to nutrient loading with nitrogen fixation significantly increased in phosphorus treatments and denitrification increased in all treatments to which nitrogen had been added. Rates of bioerosion and grazing showed no significant effects of added nutrients. ENCORE has shown that reef organisms and processes investigated in situ were impacted by elevated nutrients. Impacts were dependent on dose level, whether nitrogen and/or phosphorus were elevated and were often species-specific. The impacts were generally sub-lethal and subtle and the treated reefs at the end of the experiment were visually similar to control reefs. Rapid nutrient uptake indicates that nutrient concentrations alone are not adequate to assess nutrient condition of reefs. Sensitive and quantifiable biological indicators need to be developed for coral reef ecosystems. The potential bioindicators identified in ENCORE should be tested in future research on coral reef/nutrient interactions. Synergistic and cumulative effects of elevated nutrients and other environmental parameters, comparative studies of intact vs. disturbed reefs, offshore vs. inshore reefs, or the ability of a nutrient-stressed reef to respond to natural disturbances require elucidation. An expanded understanding of coral reef responses to anthropogenic impacts is necessary, particularly regarding the subtle, sub-lethal effects detected in the ENCORE studies.

Physiological and morphological responses of the seagrass Zostera capricorni Aschers. to light intensity

SummaryPhotosynthetic responses of the temperate seagrass, Zostera marina L., were examined by manipulations of photon flux density in an eelgrass bed in Great Harbor, Woods Hole, MA during August 1981. Sun reflectors and light shading screens were placed at shallow (1.3 m) and deep (5.5 m) stations in the eelgrass bed to increase (+35% to +40%) and decrease (-55%) ambient photon flux densities. The portion of the day that light intensities exceeding the light compensation point for Z. marina (Hcomp) and the light saturation point (Hsat) were determined to assess the impact of the reflectors and shades. The Hcomp and Hsat periods at the deep station shading screen were most strongly affected; Hcomp was reduced by 11% and Hsat was reduced by 52%. Light-saturated photosynthetic rates, dark respiration rates, leaf chlorophyll content, chlorophyll a/b, PSUO2 size, PSU density, leaf area, specific leaf area, leaf turnover times and leaf production rates were determined at the end of three sets of 1- to 2-week experiments. None of the measured parameters were affected by the photon flux density manipulations at the shallow station; however, at the deep station leaf production rates were significantly reduced under the shading screen and chlorophyll a/b ratios were higher at the reflector. These results indicate that adjustment to short-term changes in light regime in Z. marina is largely by leaf production rates. Further, the most dramatic changes in the periods of compensating or saturating photon flux densities had the greatest impact on the measured photosynthetic responses.

Effects of temperature on photosynthesis and respiration in eelgrass (Zostera marina L.)

The responses of the seagrass Zostera capricorni Aschers, to changes in light intensity were examined in flowing seawater aquaria experiments. Plants were grown in six light regimes: full sunlight (100%), 50, 30, 20, 15, and 5% of full light over a 2-month period. Measurements of growth, biomass, pigments, stable isotopes and leaf anatomy were made at the end of the experiment. Plants survived under all light treatments, even below minimum light requirements of related seagrasses. However, the experimental light levels possibly do not correspond to light reaching seagrass leaves under natural conditions. Plants grown under high light conditions (50–100% light) had smaller shoots, higher biomass and productivity, less negative δ13C values, lower leaf nitrogen content, less chlorophyll and more ultraviolet light absorbing pigment than plants grown under low light conditions (<20% light). Photoadaptation by ultraviolet light absorbing pigment(s) was noted, with more variability in ultraviolet light pigments than in chlorophyll levels. Increased CO2 demand and/or increased CO2 recycling in internal gas spaces may account for the less negative δ13C values in high light treatments, indicating less isotopic discrimination in seagrass leaves in high light. A saturation response of growth rates to light intensity was observed, with less substantial growth reductions at lower light intensities than observed in other seagrass shading experiments. Nutrient limitation in high light was inferred by a growth maximum at 50% light level, increased root biomass and lower leaf nitrogen content in high light treatments. Overall, a wide range of morphological and physiological photoadaptive responses not previously reported in Zostera capricorni was observed.

Photosynthetic responses of eelgrass (Zostera marina L.) to light and sediment sulfide in a shallow barrier island lagoon

Abstract The short-term temperature responses of the photosynthesis-irradiance (P-I) relationships and respiration of Zostera marina L. (eelgrass) leaves were determined at eight temperatures from 0 to 35°C for plants growing at 20–22°C in Great Harbor, Woods Hole, Massachusetts. Light-saturated, net photosynthesis increased with temperature up to an optimum of 25–30°C and decreased at 35°C. Dark respiration increased with temperature from 5 to 35°C. The initial slopes of the P-I curves were relatively constant between 5 and 30°C, but were greatest at 0 and least at 35°C. The photosynthetic saturation and compensation photon flux densities generally increased with increasing temperature. A Q10 value, determined over the temperature range 0–35°C, of 1.5–1.7 was obtained for light-saturated photosynthesis, while the value for dark respiration was 2.4. Ratios of maximum photosynthetic rates to respiration rates (P : R) were highest at 5°C and declined markedly at higher and lower temperatures. Calculations of daily carbon balances from P : R ratios and daily light regimes indicate that net positive leaf carbon balance could be maintained by Z. marina leaves in Great Harbor under winter temperature and light regimes, while high temperatures (⩾30°C) lead to negative daily carbon balances of leaves which could contribute to mortality or reduced growth of the plants.