Kenneth H. Dunton

Dependence of consumers on macroalgal (Laminaria solidungula) carbon in an arctic kelp community: δ13C evidence

Stable carbon isotope measurements (δ13C) were used to assess the importance of kelp carbon (-13.6 to-16.5‰) versus phytoplankton carbon (-25.5 to-26.5‰) to resident fauna of an isolated kelp bed community on Alaskas north arctic coast from 1979 to 1983. The predominant kelp, Laminaria solidungula, showed some seasonal variation in δ13C which was correlated with changes in the carbon content of the tissue. Animals that showed the greatest assimilation of kelp carbon (>=50%) included macroalgal herbivores (gastropods and chitons,-16.9 to-18.2‰), a nonselective suspension feeder (an ascidian,-19.0‰) and a predatory gastropod (-17.6‰). Animals that showed the least incorporation of kelp carbon into body tissues (<=7%) included selective suspension-feeders (hydroids, soft corals and bryozoans,-22.8 to-25.1‰). Sponges, and polychaete, gastropod and crustacean omnivores exhibited an intermediate dependence on kelp carbon (15 to 40%). Within some taxonomic groups, species exhibited a broad range in isotopic composition which was related to differences in feeding strategies. In the polychaete group alone, δ13C values identified four major feeding habits: deposit-feeders (-18.0‰), omnivores (-20.4‰), predators (-22.2‰) and microalgal herbivores (-23.0‰). Distinct seasonal changes in the δ13C values of several animals indicated an increased dependence on kelp carbon during the dark winter period when phytoplankton were absent. Up to 50% of the body carbon of mysid crustaceans, which are key prey species for birds, fishes and marine mammals, was composed of carbon derived from kelp detritus during the ice-covered period.

Effect of in situ light reduction on the maintenance, growth and partitioning of carbon resources in Thalassia testudinum banks ex König

Abstract The effects of in situ light reduction on the subtropical seagrass, Thalassia testudinum Banks ex Konig, in Corpus Christi Bay, Texas were examined from April 1993 to August 1994. The annual quantum flux at the seagrass canopy level was 5207 mol · m−2 · yr−1 or 46% of surface irradiance (SI) compared with two manipulated treatments that reduced underwater light to 1628 mol · m−2 · yr−1 (14% SI) and 864 mol · m−2 · yr−1 (5% SI). All plants subjected to 5% SI died after 200 days and over 99% of the plants receiving 14% SI died by the end of the experiment (490 days). Blade widths of plants in the control groups ranged from 6.4 to 7.0 mm, but decreased within months to 4.7 mm in both treatment group as a result of light reduction. Leaf production rates were significantly higher in control plants compared to plants within the 14 and 5% SI treatment groups, with all plants showing a seasonal trend of high productivity in July and low productivity in April. Blade chlorophyll (chl) concentrations increased, while the chl a-b ratio decreased with reduced light level. Soluble carbohydrate carbon content of controls was highest in rhizomes (102–152 mg C · g−1 dry wt.) and was relatively low in leaves (50–66 mg C · g−1 dry wt.) and in roots (57–74 mg C · g−1 dry wt.). In both light treatment groups, rhizome carbohydrate carbon content was 50% lower and leaf carbohydrate carbon content was about 15% lower than in controls, while the root carbon content did not differ significantly between the treatment groups and the controls. Ammonium and sulfide concentrations of pore water in the shaded cages were significantly higher than in control cages. We conclude that indices of shoot density, blade width, leaf growth, chl a:b ratio and blade chlorophyll content may be important early warning indicators of chronic underwater light stress in T. testudinum. A quantum flux of 1628 mol · m−2 · yr−1 (14% SI) was insufficient to maintain a positive carbon balance in T. testudinum in this bay system.

Seasonal growth and biomass of the subtropical seagrassHalodule wrightii in relation to continuous measurements of underwater irradiance

Continuous year-round measurements of photosynthetically active radiation (PAR) were collected in relation to leaf elongation and plant biomass in the shoal-grass,Halodule wrightii Aschers., within three different estuarine systems on the south Texas coast (Laguna Madre: May 1989 to September 1993; Corpus Christi Bay: February 1990 to September 1993; San Antonio Bay; May 1990 to April 1991). Large differences in water transparency at all three sites masked seasonal variations in surface insolation as reflected in average diffuse attenuation coefficient (k) values ranging from 0.7 to 2.9 and differences in the maximum depth penetration ofH. wrightii, which varied from 0.6 to about 1.3 m. The continuous presence of a chrysophyte (brown tide) algal bloom in Laguna Madre since 1990 led to significant decreases in spring leaf elongation rates and a nearly 50% decline in below-ground biomass, which was reflected in root:shoot ratio (RSR) values that declined from 5.4 in 1989 to 2.3 in 1992. Increased turbidity and lower light levels in San Antonio Bay also corresponded with diminished plant biomass and the subsequent loss of plants; at both locations, the annual quantum flux ranged from 2200 to 2400 mol m-2yr-1, or about 18% of surface irradiance (SI). In contrast,H. wrightii populations growing at ca.1.2 m depths and characterized by high RSR values (≥4.0) were exposed to 5100 to 5700 mol m-2yr-1, or about 41 to 46% SI. Under these conditions, plants were exposed to daily saturating levels of PAR (Hsat) of 3 to 8 h during the spring/summer period of maximum growth, compared to an average of 2 h in Laguna Madre (after 1990) and San Antonio Bay based on field-derived measurements of photosynthetic parameters. Leaf elongation inH. wrightii exhibited a clear circannual rhythm at all sites, regardless of underwater light levels and therefore was not a sensitive indicator of light stress. Instead, chronic long-term reductions in underwater PAR were most strongly reflected in total plant biomass. The higher light demand (18% SI) forH. wrightii in relation to many other seagrasses (11% SI; Duarte 1991) may be related to its higher photosynthetic light requirement, but may also reflect the different methods used to evaluate the minimum light requirements of seagrasses. In estuarine and coastal waters, which are characterized by large and unpredictable variations in water transparency, continuous measurements of in situ PAR are invaluable in assessing the growth and photosynthetic response of seagrasses to variations in underwater irradiance.

Arctic biogeography: The paradox of the marine benthic fauna and flora.

The marine benthic fauna and flora that inhabit the shallow arctic sublittoral zone comprise a relatively young marine assemblage characterized by species of either Pacific or Atlantic affinity and notably few endemics. The young character of nearshore arctic communities, as well as their biogeographical composition, is largely a product of the Pleistocene glaciation. However, analysis of more recent collections and comparison between the origins of the benthic fauna and flora present some interesting paradoxes to biogeographers. One enigma is the low frequency of algal species with Pacific affinities in the Arctic, especially in the Chukchi, Beaufort and East Siberian Seas of the Eastern Arctic, which receive direct inputs of northward-flowing Pacific waters. In contrast, animal species with Pacific affinities are found throughout the nearshore regions of the Arctic, reaching their highest frequency in the marginal seas between the New Siberian Islands and the Canadian Archipelago. Organization of published and unpublished data, additional field collections, and the use of cladistics and molecular DNA techniques by systematists are a high priority for future research in reconstructing the evolution of the arctic biotic assemblage.

The Arctic Ocean Estuary

Large freshwater contributions to the Arctic Ocean from a variety of sources combine in what is, by global standards, a remarkably small ocean basin. Indeed, the Arctic Ocean receives ∼11% of global river discharge while accounting for only ∼1% of global ocean volume. As a consequence, estuarine gradients are a defining feature not only near-shore, but throughout the Arctic Ocean. Sea-ice dynamics also play a pivotal role in the salinity regime, adding salt to the underlying water during ice formation and releasing fresh water during ice thaw. Our understanding of physical–chemical–biological interactions within this complex system is rapidly advancing. However, much of the estuarine research to date has focused on summer, open water conditions. Furthermore, our current conceptual model for Arctic estuaries is primarily based on studies of a few major river inflows. Future advancement of estuarine research in the Arctic requires concerted seasonal coverage as well as a commitment to working within a broader range of systems. With clear signals of climate change occurring in the Arctic and greater changes anticipated in the future, there is good reason to accelerate estuarine research efforts in the region. In particular, elucidating estuarine dynamics across the near-shore to ocean-wide domains is vital for understanding potential climate impacts on local ecosystems as well as broader climate feedbacks associated with storage and release of fresh water and carbon.

The effects of in situ light reduction on the growth of two subtropical seagrasses, Thalassia testudinum and Halodule wrightii

The effects of in situ light reductions on two species of subtropical seagrasses, Thalassia testudirum (reduced to 14% and 10% of surface irradiance; SI) and Halodule wrightii (reduced to 16% and 13% SI) were examined over a 10-mo period (October 1992-September 1993) in relation to leaf elongation rates, sediment pore-water ammonium, and blade chlorophyll concentrations. No significant changes in pore-water ammonium levels were noted among treatments with time, but blade chlorophyll concentrations in both species were higher in plants exposed to the darkest treatments (10% and 13% SI) relative to controls exposed to 50% SI. In all treatments, blade chlorophyll concentrations were highest and chlorophyll a:b ratios lowest during the warner months, coincident with higher water temperatures. Leaf elongation rates in T. testudinum plants decreased relative to unshaded controls after 1 mo of treatment in autumn, but no significant differences in leaf elongation were noted among treatments for H. wrightii in late autumn or winter when very low growth rates (<0.1 cm shoot−1 d−1) were recorded. There were no differences between treatments during the spring growth period for T. testudinum (no data are available for H. wrightii), suggesting that growth (ca. 1 cm shoot−1 d−1) was probably not related to available light but was supported by belowground reserves. After 10 mo of treatment, all H. wrightii plants at 13% SI (1,600 mol m−2 yr−1) and 16% SI (2,000 mol m−2 yr−1) disappeared from experimental plots; similarly, no T. testudinum plants exposed to 10% SI (1,300 mol m−2 yr−1) remained, although 4% of the plants at 14% SI (1,800 mol m−2 yr−1) survived nearly 12 mo of reduced irradiance. In neither species were leaf elongation rates, which showed little change among treatments, a reliable indicator of the underwater light environment.

A SEASONAL COMPARISON OF CARBON, NITROGEN, AND PIGMENT CONTENT IN LAMINARIA SOLIDUNGULA AND L. SACCHARINA (PHAEOPHYTA) IN THE ALASKAN ARCTIC1

Laminaria solidungula and L. saccharina inhabit the Beaufort Sea in the Alaskan High Arctic. Laminaria solidungula is an Arctic endemic, whereas L. saccharina extends from north temperate Pacific and Atlantic waters to the Arctic. Previous studies have shown that the two species have different seasonal timing of growth, but little comparative physiological information exists. As a first step in characterizing these two species from a mixed Arctic population, we measured variations in carbon, nitrogen, and photosynthetic pigment content in blade tissue from plants collected under the fast ice in April and during the open water Period in late July, Both species exhibited seasonal differences in many measured variables; seasonal differences in L. solidungula were most pronounced in growing basal blades. For example, the molar CIN ratio of basal blades averaged 11 in April and 21 in July for L. solidungula and 11.5 in April and 28 in July for L. saccharina. Basal and mature second blades differed in pigment content in April but not in July: chlorophyll a + c in L. solidungula basal and mature second blades averaged 19 and 27 nmol.cm−2 in April and 30 and 29 nmol. cm−2 in July, respectively. The corresponding values for L. saccharina were 17 and 29 nmol.cm−2 in April and 16 and 16 nmol.cm−2 in July (95% confidence intervals approximately 1–3 nmol. cm −2). Carotenoids exhibited similar patterns. Species differences in pigments, carbon, and nitrogen were minor and were probably effects rather than causes of the different seasonal patterns of growth and development. The primary difference between the two species may be the ability of L. solidungula to retain multiple metabolically active blades and to fuel areal growth with stored carbohydrates during winter near‐darkness, whereas L. saccharina growth is more closely tied to active photosynthesis in the growing basal blade. The cause of old blade retention in L. solidungula and the possibility of other physiological differences between the two species, including gametophytes, remain to be determined.

Diurnal changes in pore water sulfide concentrations in the seagrass Thalassia testudinum beds : the effects of seagrasses on sulfide dynamics

The dynamics of the seagrass-sulfide interaction were examined in relation to diel changes in sediment pore water sulfide concentrations in Thalassia testudinum beds and adjacent bare areas in Corpus Christi Bay and lower Laguna Madre, Texas, USA, during July 1996. Pore water sulfide concentrations in seagrass beds were significantly higher than in adjacent bare areas and showed strong diurnal variations; levels significantly decreased during mid-day at shallow sediment depths (0-10 cm) containing high below-ground tissue biomass and surface area. In contrast, diurnal variations in sediment sulfide concentrations were absent in adjacent bare patches, and at deeper (>10 cm) sediment depths characterized by low below-ground plant biomass or when the grasses were experimentally shaded. These observations suggest that the mid-day depressions in sulfide levels are linked to the transport of photosynthetically produced oxygen to seagrass below-ground tissues that fuels sediment sulfide oxidation. Lower sulfide concentrations in bare areas are likely a result of low sulfate reduction rates due to low organic matter available for remineralization. Further, high reoxidation rates due to rapid exchange between anoxic pore water and oxic overlying water are probably stimulated in bare areas by higher current velocity on the sediment surface than in seagrass beds. The dynamics of pore water sulfides in seagrass beds suggest no toxic sulfide intrusion into below-ground tissues during photosynthetic periods and demonstrate that the sediment chemical environment is considerably modified by seagrasses. The reduced sediment sulfide levels in seagrass beds during photosynthetic periods will enhance seagrass production through reduced sulfide toxicity to seagrasses and sediment microorganisms related to the nutrient cycling.

Light and temperature demands of marine benthic microalgae and seaweeds in polar regions

Polar algae have a striking ability to photosynthesize and grow under very low light and temperatures. In seaweeds, minimum light demands for photosynthetic saturation and compensation can be as low as 10 and 2 mu mol photons m(-2) s(-1), respectively. For benthic microalgae, these values can be even lower because of the limited irradiance reaching deep sea floors. The extreme shade adaptation of these organisms sets their distributional limits at depths close to 40 m and enables them to tolerate long periods of extended darkness. In addition to their capability for efficient photosynthesis at extremely low light levels, polar algae possess metabolic adaptations to persist at low temperatures, which permit them to complete their life cycles at year-round temperatures close to 0 degrees C. Seaweeds with the lowest temperature demands are the species endemic to the Antarctic while Arctic algae are comparatively less cold-adapted. These adaptive characteristics allow benthic marine algae to make high contributions to high latitude coastal primary productivity and energy fluxes, exceeding or equaling the production of primary producers in more temperate systems. The studies summarized here give important insights into the major physiological adaptations allowing marine benthic microalgae and seaweeds to colonize these extreme habitats.

production ecology of Ruppia maritima L. s.l. and Halodule wrightii Aschers, in two subtropical estuaries

Abstract Seasonal growth and production dynamics of Ruppia maritima L. s.l. were compared over a 3-year period in two south Texas estuaries characterized by different salinity and N regimes as a result of freshwater inputs. Measurements of shoot production in the Guadalupe Estuary (0–25‰ salinity) and the Nueces Estuary (32–38‰ salinity) revealed no major differences in the magnitude of growth, but the plant populations differed in the seasonality of growth, the time of flowering, and the persistence of an over-wintering population. During the period of rapid shoot development, from March to August, leaf elongation rates usually ranged from 2 to 4 mm·day −1 (0.04–0.08 mg dry wt·mg shoot −1 ·day −1 ), although peak growth rates of up to 8 mm·day −1 were also recorded. In both estuaries, R. maritima was a strict opportunist, yearly colonizing bare areas and completing its entire growth cycle in 4 months. Overwintering populations of R. maritima existed at the lower nutrient and higher salinity site in Nueces Estuary but not at the high nutrient, low salinity site in Guadalupe Estuary. Halodule wrightii Aschers, was absent from Guadalupe Estuary, but in the Nueces Estuary, H. wrightii maintained large and persistent overwintering populations characterized by sustained year-round growth and larger below-ground root and rhizome fractions of total biomass (50–85%) compared to R. maritima (20–70%). The high nutrient regimes associated with large inputs of freshwater in the Guadalupe Estuary appear to have little beneficial effect on the growth dynamics of R. maritima . Instead, significant reductions in underwater light may be most important for growth. Fouling by algal epiphytes, higher riverine inflow, and local physiographic differences in wave exposure appear to be the primary factors regulating light levels and thus the proliferation and year-round persistence of R. maritima .