Michael J. Durako

Characterization of CDOM in an organic rich river and surrounding coastal ocean in the South Atlantic Bight

Abstract.Variability in chromophoric dissolved organic carbon (CDOM) was characterized in and around the Cape Fear River and Onslow Bay, North Carolina USA. The river end member of the study is extremely rich in CDOM, thus the Cape Fear River serves as a point source of CDOM-rich water into the southeastern Atlantic bight. The river plume is easily traceable and generally extends in a southwesterly direction along the coastline into Long Bay. Depending on physical processes and river flow, the plume can meander somewhat and may even turn northward for short periods of time. The oceanic end member of this study was the Gulf Stream. Samples from the Gulf Stream were obtained up to 97 km off shore. The experimental approach focused on the qualitative and quantitative description of CDOM from fresh-to-oceanic waters. CDOM was characterized by excitation emission matrix (EEM) fluorescence and UV/VIS spectroscopy. Variability of CDOM absorption in the relatively small area of the Cape Fear River estuary and surrounding coastal ocean was very high. The observed range of variability of CDOM absorption coefficient, aCDOM(350), extended over nearly the entire range of CDOM absorption in the literature: 0.046 = aCDOM(350) = 29.9 m–1. Changes in CDOM absorption spectrum slope coefficient S, were small in the Cape Fear River plume area, but relatively large in Onslow Bay. CDOM EEM spectra indicated that a radical change in composition of CDOM occurs along the river-to-oceanic salinity gradient. CDOM in the coastal ocean was characterized by strong reduction of the three principal intensity peaks: A, C, and M, and a prominent contribution of the T peak to the fluorescence spectrum. The fluorescence intensity is linearly related with absorption. There is a strong inverse relationship between salinity and CDOM absorption. The distribution of the slope coefficient and the percent contribution of respective peak intensities to the total EEM intensity showed that CDOM undergoes conservative mixing until it approached oceanic salinity. Thus, CDOM is so concentrated in the river that mixing and other physical processes mask photochemical or biological alteration of its composition.

Physiological effects of short-term salinity changes on Ruppia maritima

Changes in Ruppia maritimaL. leaf-tissue osmolality, compatible solute synthesis in leaf tissues, and maximum effective quantum yield in response to short-term changes in salinity were investigated. Plants cultured at 20‰ S were exposed to 0‰ S, 10‰ S (half-ambient), 20‰ S (ambient), and 40‰ S (twice-ambient) salinities. Total and non-vacuolar leaf osmolality for cultured plants significantly decreased (total: from 1464 ± 266 to 712± 210 mmol kg −1 ; non-vacuolar: from 880 ± 108 to 257 ± 80 mmol kg −1 ) or increased (total: from 1464 to 2532 ± 673 mmol kg −1 , non-vacuolar from 880 to 1168 ± 15 mmol kg −1 ), within 1 min of exposure to 0 and 40‰ S, respectively. After the initial rapid change in leaf osmolality, values were relatively constant for the first 180 min of exposure. Osmolality then changed again over the period from 1 to 2 days post-treatment with values again increasing (40‰ S: total = 3152 ± 335 mmol kg −1 , non-vacuolar = 1967 ± 103 mmol kg −1 )o r decreasing (0‰ S: total = 357± 46 mmol kg −1 , non-vacuolar = 74± 32 mmol kg −1 ) with salinity. Soluble and total carbohydrates in leaf tissues responded differently to changing salinity. Total carbohydrates decreased by 65%, while soluble levels increased by 34%, in high salinity. Total and soluble proline levels increased (63 and 18%, respectively), and decreased (−36 and −20% for 10‰ S; −72 and −32% for 0‰ S, respectively), with salinity. These results suggest that both proline and soluble carbohydrates act as compatible solutes. Maximum quantum yields (Fv/Fm) were measured over a 48 h period in response to changes in medium salinity and inorganic carbon (ambient and ∼2.0 mM bicarbonate-equilibrium treatments). Fv/Fm exhibited significant variation in response to salinity, bicarbonate-level and time as main effects, with significant interactions. Quantum yields were lowest in the 0 and 40‰ S treatments; the 10 and 20‰ S treatments had significantly higher quantum yields. These short-term responses indicated that both increases and reductions of external ion concentrations are initially stressful for R. maritima, but that it can physiologically adjust after several days.

Decadal changes in seagrass distribution and abundance in Florida Bay

The Florida Bay ecosystem has changed substantially in the past decade, and alterations in the seagrass communities have been particularly conspicuous. In 1987 large areas ofThalassia testudinum (turtlegrass) began dying rapidly in western Florida Bay. Although the rate has slowed considerably, die-off continues in many parts of the bay. Since 1991, seagrasses in Florida Bay have been subjected to decreased light availability due to widespread, persistent microalgal blooms and resuspended sediments. In light of these recent impacts, we determined the current status of Florida Bay seagrass communities. During the summer of 1994, seagrass species composition, shoot density, shoot morphometrics, and standing crop were measured at 107 stations. Seagrasses had been quantified at these same stations 10 yr earlier by Zieman et al. (1989).T. testudinum was the most widespread and abundant seagrass species in Florida Bay in both 1984 and 1994, and turtlegrass distribution changed little over the decade. On a baywide basis,T. testudinum density and biomass declined significantly between surveys; mean short-shoot density ofT. testudinum dropped by 22% and standing crop by 28% over the decade.T. testudinum decline was not homogeneous throughout Florida Bay; largest reductions in shoot density and biomass were located principally in the central and western bay. Percent loss ofT. testudinum standing crop in western Florida Bay in 1994 was considerably greater at the stations with the highest levels of standing crop in 1984 (126–215 g dry wt m−2) than at the stations with lower levels of biomass. While turtlegrass distribution remained consistent over time, both the distribution and abundance of two other seagrasses,Halodule wrightii andSyringodium filiforme, declined substantially between 1984 and 1994. Baywide,H. wrightii shoot density and standing crop declined by 92%, andS. filiforme density and standing crop declined by 93% and 88%, respectively, between surveys. Patterns of seagrass loss in Florida Bay between 1984 and 1994 suggest die-off and chronic light reductions were the most likely causes for decline. If die-off and persistent water-column turbidity continue in Florida Bay, the long-term future of seagrasses in the bay is uncertain.

FORECASTING RESPONSES OF SEAGRASS DISTRIBUTIONS TO CHANGING WATER QUALITY USING MONITORING DATA

Extensive data sets on water quality and seagrass distributions in Florida Bay have been assembled under complementary, but independent, monitoring programs. This paper presents the landscape-scale results from these monitoring programs and outlines a method for exploring the relationships between two such data sets. Seagrass species occurrence and abundance data were used to define eight benthic habitat classes from 677 sampling locations in Florida Bay. Water quality data from 28 monitoring stations spread across the Bay were used to construct a discriminant function model that assigned a prob- ability of a given benthic habitat class occurring for a given combination of water quality variables. Mean salinity, salinity variability, the amount of light reaching the benthos, sediment depth, and mean nutrient concentrations were important predictor variables in the discriminant function model. Using a cross-validated classification scheme, this discrimi- nant function identified the most likely benthic habitat type as the actual habitat type in most cases. The model predicted that the distribution of benthic habitat types in Florida Bay would likely change if water quality and water delivery were changed by human engineering of freshwater discharge from the Everglades. Specifically, an increase in the seasonal delivery of freshwater to Florida Bay should cause an expansion of seagrass beds dominated by Ruppia maritima and Halodule wrightii at the expense of the Thalassia testudinum-dominated community that now occurs in northeast Florida Bay. These statistical techniques should prove useful for predicting landscape-scale changes in community com- position in diverse systems where communities are in quasi-equilibrium with environmental drivers.

Photosynthetic characteristics of Thalassia testudinum measured in situ by pulse-amplitude modulated (PAM) fluorometry: methodological and scale-based considerations

Abstract Shoot-to-landscape scale sources of variation in photosynthetic characteristics of Thalassia testudinum, as measured in situ using a submersible pulse-amplitude modulated fluorometer (diving-PAM), were investigated. Shoot-scale variation was assessed to develop a standard-methods protocol for this species. Significant within-shoot and among-shoot scale variation was observed for several PAM-fluorescence parameters. The fraction of incident photosynthetically active radiation (PAR) absorbed by rank 1 (youngest) leaves, 0.67±0.03, was significantly lower than the PAR absorbed by rank 2 and 3 leaves (0.78±0.04 and 0.77±0.04, respectively). Quantum yields (Y) and photosynthetic efficiency (Fv/Fm) exhibited greater variability and generally decreased with increasing leaf age (Y=0.78±0.02, 0.78±0.03, and 0.76±0.06 and Fv/Fm=0.79±0.02, 0.77±0.5, and 0.75±0.06 for rank 1, 2, and 3 leaves, respectively). Maximum fluorescence of light-acclimated leaves (F′m) significantly decreased from the base to the tip of leaves (1032±203, 1059±139, and 793±107 for the base, middle and tip of the leaf) and Y was significantly reduced from 0.68–0.76 to 0.27–0.62 when measured where leaf lesions were present. However, paired comparisons (with versus without lesions at the tip of the leaf) for the base, middle, and tip areas of rank 2 leaves indicated that lesions effects on photosynthesis were localized only to the area of the lesion (tip). The Y exhibited relatively high variability for short-shoots within seagrass die-off patches and were significantly reduced for short-shoots exhibiting die-off symptoms (rotten leaf base grayish-green leaf color, but no lesions). Negative response slopes for Y and Fv/Fm, measured as part of a landscape-scale sampling program, revealed the presence of significant diurnal (time-of-day) and photosynthetic photon flux density (PPFD) related variation, with a more negative slope for Y. These results indicate that the selection of leaf tissue, short-shoot location, and time of measurement need to be considered when determining photosynthetic rates for seagrasses in situ.

Ecology of seagrasses in northeastern Saudi Arabia one year after the Gulf War oil spill

Approximately 1 year after the Gulf War oil spill we examined the distribution, species composition, abundance and productivity of seagrasses in several oil contaminated bays along the northeastern coastline of Saudi Arabia. The most abundant seagrass, Halodule uninervis, grew from the intertidal down to depths of 10 m with a standing crop ranging from 2 to 32 gdw m−2. Specific leaf productivity in a heavily oiled shallow site (1.5 m depth) was 2.2% d−1 and areal production was 0.172 gdw m−2 d−1. Specific productivity was similar to other reported rates for healthy populations of Halodule species but the low areal productivity reflected late winter water temperatures which were between 16 and 19°C. Halophila ovalis was intermediate in abundance followed by Halophila stipulacea. Leaf morphology and indicators of vegetative growth suggested that all three species were healthy, despite the recent history of oiling. Morphometric analyses indicated the seagrasses were experiencing a seasonal transition and initializing a normal growth pattern from lower winter temperatures to spring conditions. All three species grew in monospecific and mixed species meadows. The highest diversity and biomass of vegetation occurred on a mixture of hard and soft substrates where species of brown macroalgae were dominant. Based on these observations we conclude that seagrasses in the northwestern Gulf have not experienced acute or long-term degradation as a direct result of the Gulf War oil spill.

Variations in water clarity and bottom albedo in Florida Bay from 1985 to 1997

Following extensive seagrass die-offs of the late 1980s and early 1990s, Florida Bay reportedly had significant declines in water clarity due to turbidity and algal blooms. Scant information exists on the extent of the decline, as this bay was not investigated for water quality concerns before the die-offs and limited areas were sampled after the primary die-off. We use imagery from the Advanced Very High Resolution Radiometer (AVHRR) to examine water clarity in Florida Bay for the period 1985 to 1997. The AVHRR provides data on nominal water reflectance and estimated light attenuation, which are used here to describe turbidity conditions in the bay on a seasonal basis. In situ observations on changes in seagrass abundance within the bay, combined with the satellite data, provide additional insights into losses of seagrass. The imagery shows an extensive region to the west of Florida Bay having increased reflectance and light attenuation in both winter and summer begining in winter of 1988. These increases are consistent with a change from dense seagrass to sparse or negligible cover. Approximately 200 km2 of these offshore seagrasses may have been lost during the primary die-off (1988 through 1991), significantly more than in the bay. The imagery shows the distribution and timing of increased turbidity that followed the die-offs in the northwestern regions of the bay, exemplified in Rankin Lake and Johnson Key Basin, and indicates that about 200 km2 of dense seagrass may have been lost or severely degraded within the bay from the start of the die-off. The decline in water clarity has continued in the northwestern bay since 1991. The area west of the Everglades National Park boundaries has shown decreases in both winter turbidity and summer reflectances, suggestive of partial seagrass recovery. Areas of low reflectance associated with a majorSyringodium filiforme seagrass meadow north of Marathon (Vaca Key, in the Florida Keys) appear to have expanded westward toward Big Pine Key, indicating changes in the bottom cover from before the die-off. The southern and eastern sections of the Bay have not shown significant changes in water clarity or bottom albedo throughout the entire time period.

Factors affecting the reproductive ecology of Thalassia testudinum (Hydrocharitaceae)

Abstract Morpho-anatomical studies of short-shoots of Thalassia testudinum Banks ex Konig collected in Tampa Bay, Florida from February 1979 to October 1980 ( n = 5394) demonstrated the presence of sexually reproductive short-shoots during all months, except August and September. Mean inflorescence lengths (MIL) indicated that little inflorescence development occurs during autumn and winter (MIL 0.2–1.0 mm); MILs in 1979 increased rapidly during April (19.47 mm ± 17.04 mm SD) and May (74.10 mm ± 27.11 mm SD), peaking in June (110.17 mm ± 8.88 mm SD). Large standard deviations reflect the presence of early stage inflorescences throughout the reproductive season. This suggests possible genetic diversity and aseasonality for floral induction. Quantitative in situ observations from 1981 to 1985 detected significant temporal and spatial variations in reproductive patterns. Annual variations in flowering (anthesis) may be explained by the influence of environmental factors, such as water temperature and salinity, on inflorescence development and survival. Spatial patchiness in reproductive effort, negative correlations between male and female short-shoot distributions and sex-related differences in the number of leaf scars short-shoot −1 may reflect clonal variation and/or a possible interaction of ramet age with sex expression in this species.

Assessment of the toxicity of Kuwait crude oil on the photosynthesis and respiration of seagrasses of the northern Gulf

Photosynthetic and respiratory responses of leaf tissues of the seagrasses Halophila ovalis, H. stipulacea, and Halodule uninervis exposed for 12–18 h to unweathered Kuwait crude oil were measured using an oxygen electrode system to assess the possibility of acute toxicity. Leaf tissues were incubated in natural seawater (control treatment) or in the water-soluble fraction of a 1% (weight:volume) solution of Kuwait crude oil in seawater (oil treatment). Photosynthesis vs. irradiance (PI) responses exhibited typical light-saturation kinetics. One-way analysis of variance detected no significant treatment effects on the PI characteristics: α, Pmax, Ik, or Ic. Respiration rates were also not significantly affected by short-term exposure to the oil treatment. In addition, no significant among-species differences in PI characteristics or respiration were detected, possibly reflecting the low metabolic state for these subtropical species during Leg II sampling when water temperatures were 16–19°C. These results support our observations that the Gulf War oil spill primarily impacted intertidal communities rather than the submergent plant communities of the northern Gulf region.

Distribution, abundance, and species composition of seagrasses at several sites in Oman

Abstract Distribution, abundance, and biomass data for seagrass communities at several locations on the coast of Oman are presented. The main study site was on the western side of Masirah Island on the Arabian Sea coast of Oman. This area is an important feeding ground for the green turtle, Chelonia mydas L., and it is affected by upwelling of low temperature waters during the summer monsoon. The depth distributions of Halodule uninervis (Forssk.) Aschers. and Halophila ovalis (R. Brown) Hook. f. , the two most abundant seagrasses at this site, overlapped but were inversely related. Halodule dominated the intertidal zone and Halophila was more predominant in the deep subtidal, although total biomass of the two seagrasses were similar in this depth zone. At all depths, biomass of Halophila was about equally distributed between leaves and roots and rhizomes. Leaf biomass of Halodule was only 7–20% of the total biomass and the highest below-ground biomass occurred in the intertidal zone. Biomass of these species here and at other sites and of Thalassodendron ciliatum (Forssk.) den Hartog at this site was generally lower than comparative data in the Gulf and the Red Sea. Small patches of Syringodium isoetifolium (Aschers.) Dandy were also observed in Umm ar Rasas Bight making a total of four species recorded to occur in Oman. The reduced growth of seagrasses at Masirah Island seems to be due to stresses associated with the summer monsoon and grazing pressure. Survival of these populations is discussed in terms of seasonal growth and flowering.