Sources of uncertainty in estimation of eelgrass depth limits

In coastal areas seagrasses have considerable ecological importance and respond to eutrophication pressures. Seagrasses have, therefore, become an important parameter for assessing ecological status of marine water bodies. In this study we analyzed the sources of uncertainty associated with the monitoring of the maximum depth limit of eelgrass (Zostera marina). Based on a long-term marine monitoring of eelgrass depth limit in Danish coastal waters we estimated the uncertainty contribution of years, divers, transects, and replicates in water bodies and their sub-areas. For all these components the absolute uncertainty increased with the maximum depth limit. We used either a Spheric or a Gaussian function to describe the relationship between uncertainty and the maximum depth limit for each variable. This parameterization of the depth-specific uncertainty allowed estimation of the total variance associated with monitoring, which can be used to evaluate monitoring designs. The variance components were included in a time budget for optimizing eelgrass monitoring. With a maximum time limit of 100 or 200 h allocated for monitoring, the design that resulted in the lowest variance of the estimated maximum depth limit used 2 or 3 divers, respectively, and involved 2 or 3 years of monitoring and 4–8 transects.     

Lake optics and depth limits for photogenesis and photosynthesis in charophyte meadows

Two series of lakes with increasing attenuation were examined for trends in spectral composition. They became the basis for an evaluation of the light environment at the lower boundary (LB) of Nitella meadows in three other series of lakes. Increased attenuation (K _d PAR) was marked by progressive erosion of the blue window and caused primarily by humic substances. An increase in K _d PAR from 0.06 to 0.81 produced, at the floor of the euphotic zone, a shift in K _d min from 440 to 580 nm. Regressions of boundary depths of Nitella meadows on water clarity produced similar slope coefficients for the three series of lakes. Several trends became evident: 1, PAR irradiance at the LB increases with depth of the LB; 2, red light (E _d 660) declines from richness at shallow LB to near extinction in deep water LB in clear lakes; while 3, blue light (K _d 450) increases to an asymptote. Blue light appears to substitute, although less effectively, for red light irradiance in the growth regulation of charophytes. These data support an hypothesis that spectral quality is involved in the determination of lower boundary depths for benthic macro-algae.     

Macrophyte depth limits in North Island (New Zealand) lakes of differing clarity

The seasonal variation in water clarity, as indicated by the attenuation coefficient for photosynthetically active radiation, K _d (m^-1), was determined by monthly measurements for a year in 9 North Island, New Zealand lakes. K _d varied by a factor of 2 to 3 in 8 of the lakes, and a factor of 5 in one. Annual mean K _d (symbol% MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaam4saaaa!36BD!\[K\] _d) varied by a factor of approximately 15 between lakes. The maximum depth of water colonized by macrophytes (z _c)was also determined. Values of z _c were in the range 1.5–12.5 m. The relationship z _c =4.34/% MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaam4saaaa!36BD!\[K\]_d accounted for most (93 percent) of the variability in z _c , indicating that average annual clarity was probably a useful predictor of z _c in lakes in this region. The values of z _c in these North Island lakes were generally greater than values calculated using previously published empirical relationships derived for northern hemisphere groups of lakes. The extent to which these relationships underestimated z _c in the North Island lakes was broadly related to latitude. Estimated average irradiance at z _c in each lake was similar to compensation point irradiances reported previously for freshwater macrophytes.     

Cuttlebone morphology limits habitat depth in eleven species of Sepia (Cephalopoda: Sepiidae)

The cuttlebone is a rigid buoyancy tank that imposes a depth limit on Sepia, the only living speciose cephalopod genus with a chambered shell. Sections of 59 cuttlebones from a geographically diverse sample of 11 species were examined using confocal microscopy. Sepia species that live at greater depths had thicker septa and less space between pillars than did shallow species. A plate theory analysis of cuttlebone strength based on these two measures predicted maximum capture depths accurately in most species. Thus cuttlebone morphology confers differing degrees of strength against implosion from hydrostatic pressure, which increases with increasing habitat depth. Greater strength may come at the cost of increased cuttlebone density, which impinges on the cuttlebone's buoyancy function.     

Changes in Seagrass Species Composition in Northwestern Gulf of Mexico Estuaries: Effects on Associated Seagrass Fauna

The objective of this study was to measure the communities associated with different seagrass species to predict how shifts in seagrass species composition may affect associated fauna. In the northwestern Gulf of Mexico, coverage of the historically dominant shoal grass (Halodule wrightii) is decreasing, while coverage of manatee grass (Syringodium filiforme) and turtle grass (Thalassia testudinum) is increasing. We conducted a survey of fishes, crabs, and shrimp in monospecific beds of shoal, manatee, and turtle grass habitats of South Texas, USA to assess how changes in sea grass species composition would affect associated fauna. We measured seagrass parameters including shoot density, above ground biomass, epiphyte type, and epiphyte abundance to investigate relationships between faunal abundance and these seagrass parameters. We observed significant differences in communities among three seagrass species, even though these organisms are highly motile and could easily travel among the different seagrasses. Results showed species specific relationships among several different characteristics of the seagrass community and individual species abundance. More work is needed to discern the drivers of the complex relationships between individual seagrass species and their associated fauna.     

Optimising Seagrass Conservation for Ecological Functions

Ecosystems - Animals are central to numerous ecological processes that shape the structure and function of ecosystems. It follows that species that are strongly linked to specific functions can...     

Seagrass patch size affects fish responses to edges

1.  Patch area and proximity of patch edge can influence ecological processes across patchy landscapes and may interact with each other. Different patch sizes have different amounts of core habitat, potentially affecting animal abundances at the edge and middle of patches. In this study, we tested if edge effects varied with patch size.
2.  Fish were sampled in 10 various-sized seagrass patches (114–5934 m²) using a small (0·5 m²) push net in three positions within each patch: the seagrass edge, 2 m into a patch and in the middle of a patch.
3.  The two most common species showed an interaction between patch size and the edge–interior difference in abundance. In the smallest patches, pipefish ( Stigmatopora nigra ) were at similar densities at the edge and interior, but with increasing patch size, the density at the edge habitat increased. For gobies ( Nesogobius maccullochi ), the pattern was exactly the opposite.
4.  This is the first example from a marine system of how patch size can influence the magnitude and pattern of edge effects.
5.  Both patch area and edge effects need to be considered in the development of conservation and management strategies for seagrass habitats.     

Seagrass growth and patch dynamics: cross-scale morphological plasticity

By examining the spatial distribution of rhizome morphological characteristics of the seagrass Halodule wrightii, in relation to a seasonal pattern of seagrass patch dynamics, we attempted to derive a mechanistic explanation for the variety of changes exhibited by seagrass patch shapes. Rhizome morphological characteristics (mean internodal distance, branching frequency and biomass) were measured at three spatially-recognized regions (Flood edge, Center, Ebb edge) of 5 seagrass patches, reflecting position relative to hydrodynamic flow. In addition, maps (1 resolution) of the seagrass patches were used to quantify changes in seagrass patch margins across the growing season. Rhizome morphological characteristics varied with spatial position: longer internodal distances were recorded on both edges of the patch relative to patch center, and rhizomes from Flood edges exhibited longer internodes than plants on the Ebb edge of patches. Branching frequency showed no spatially-explicit distribution across the seagrass patches. Patch change analysis indicated a pattern of increase in patch area on the Flood edges of seagrass patches and recession (or no change) on the Ebb edges. Patch margin change was significantly correlated with internodal distances: the more positive the increase in patch seagrass coverage on an edge, the greater the internodal distances.Sediment nutrients were explored as a potential mechanism for the distinct spatial distribution of morphologies found; experimental addition of phosphorus, but not nitrogen, significantly altered the rhizome morphology and biomass, but measurement of ambient sediment nutrient concentrations produced no significant correlations with the in situ distribution of rhizome morphologies. These results suggest that larger-scale landscape characteristics of patch dynamics may be determined by predictable behaviors of small-scale components, but the results do not conclusively describe a mechanism for this system.     

Consumer Depletion Alters Seagrass Resistance to an Invasive Macroalga

Few field studies have investigated how changes at one trophic level can affect the invasibility of other trophic levels. We examined the hypothesis that the spread of an introduced alga in disturbed seagrass beds with degraded canopies depends on the depletion of large consumers. We mimicked the degradation of seagrass canopies by clipping shoot density and reducing leaf length, simulating natural and anthropogenic stressors such as fish overgrazing and water quality. Caulerpa racemosa was transplanted into each plot and large consumers were excluded from half of them using cages. Potential cage artifacts were assessed by measuring irradiance, scouring by leaf movement, water flow, and sedimentation. Algal invasion of the seagrass bed differed based on the size of consumers. The alga had higher cover and size under the cages, where the seagrass was characterized by reduced shoot density and canopy height. Furthermore, canopy height had a significant effect depending on canopy density. The alteration of seagrass canopies increased the spread of C. racemosa only when large consumers were absent. Our results suggest that protecting declining habitats and/or restoring fish populations will limit the expansion of C. racemosa. Because MPAs also enhance the abundance and size of fish consuming seagrass they can indirectly promote algal invasion. The effects of MPAs on invasive species are context dependent and require balancing opposing forces, such as the conservation of seagrass canopy structure and the protection of fish grazing the seagrass.     

Trophic Transfers from Seagrass Meadows Subsidize Diverse Marine and Terrestrial Consumers

In many coastal locations, seagrass meadows are part of a greater seascape that includes both marine and terrestrial elements, each linked to the other via the foraging patterns of consumers (both predators and herbivores), and the passive drift of seagrass propagules, leaves, roots and rhizomes, and seagrass-associated macroalgal detritus. With seagrasses declining in many regions, the linkages between seagrass meadows and other habitats are being altered and diminished. Thus, it is timely to summarize what is known about the prevalence and magnitude of cross-habitat exchanges of seagrass-derived energy and materials, and to increase awareness of the importance of seagrasses to adjacent and even distant habitats. To do so we examined the literature on the extent and importance of exchanges of biomass between seagrass meadows and other habitats, both in the form of exported seagrass biomass as well as transfers of animal biomass via migration. Data were most abundant for Caribbean coral reefs and Australian beaches, and organisms for which there were quantitative estimates included Caribbean fishes and North American migratory waterfowl. Overall, data from the studies we reviewed clearly showed that seagrass ecosystems provide a large subsidy to both near and distant locations through the export of particulate organic matter and living plant and animal biomass. The consequences of continuing seagrass decline thus extend far beyond the areas where seagrasses grow.     

Further Characteristics of Salt-Dependent Bicarbonate Use by the Seagrass Zostera muelleri

Millhouse, J. and Strother, S. 1987. Further characteristicsof salt-dependent bicarbonate use by the seagrass Zostera muelleri.—J.exp. Bot. 38: 1055–1068. The contribution of HCO^–₃to photosynthetic O₂ evolutionin the seagrass Zostera muelleri Irmisch ex Aschers. increasedwith increasing salinity of the bathing seawater when the inorganiccarbon concentration was kept constant. K_1/2 (seawater salts)for HCO^–₃ -dependent photosynthesis was 66% of seawatersalinity. Both short- and long-term pretreatment at low salinitiesstimulated photosynthesis in full strength seawater. Twentyfour hours pre-incubation of seagrass plants in 3·0 molm^–3 NaHCO₃ resulted in increased photosynthesis at allsalinities, apparently due to stimulation of HCO^–₃ use(K_1/2 (seawater salts) = 26%). V_max (HCO^–₃) was not affectedby low salinity pretreatment. The kinetics of HCO^–₃ stimulationby the major seawater cations was investigated. Ca²⁺ was themost effective cation with the highest V_max (HCO^–₃) andwith K_1/2(Ca²⁺) = 14 mol m^–3. Mg²⁺ was also very effectiveat less than 50 mol m^–3 but higher concentrations wereinhibitory. This inhibition cannot be accounted for solely byprecipitation of MgCO₃. Na⁺ and K⁺ were both capable of stimulatingHCO^–₃ use. Stimulation was in two distinct parts. Up to500 mol m^–3, both citrate and chloride salts gave similarresults (K_1/2(Na⁺) 81 mol m^–3, V_max(HCO^–₃) 0·26µmol O₂ mg^–1 chl min^–1), but use of citratesalts above 500 mol m^–2 caused a second stimulation ofHCO^–₃ use (K_1/2(Na⁺) 830 mol m^–3, V_max(HCO^–₃)0·68 µmol O₂ mg^–1 chl min^–1). V_max(HCO^–₃)for the second-phase Na⁺ or K⁺ stimulation was of the same orderas for Ca²⁺-stimulated HCO^–₃ use. To further characterizesalt-dependent HCO^–₃ use, the sensitivity of photosynthesisto Tris and TES buffers was investigated. The effects of Trisappear to be due to the action of Tris⁺ causing stimulationof HCO^–₃ -dependent photosynthesis in the absence of salt,but inhibition of HCO^–₃ use in saline media. TES has noeffect on photosynthesis. External carbonic anhydrase, althoughimplicated in salt-dependent HCO^–₃ use in Z. muelleri,could not be detected in whole leaves. Key words: Zostera muelleri, HCO^–₃ use, salinity     

Light and pressure in two freshwater lakes and their influence on the growth, morphology and depth limits of Hippuris vulgaris

SUMMARY. Hippuris vulgaris was found growing down to a depth of 6 m in two clear-water lakes. Mean summer water temperatures of the epilimnia were 15–18°C. Midsummer photosynthetically available radiation, (PAR 400–700 nm) at 6 m was 100 μeinsteins m⁻²s⁻¹. In the laboratory, shoots of H. vulgaris continued to elongate at temperatures of 15 and 20°C and at pressures of 1.0, 1.8 and 2.3 atm. (corresponding to 0 m, 8.0 m and 13.3 m depths of water, respectively) providing PAR was kept above 100 °E m⁻²s⁻¹. Leaf primordia were initiated further from the apex in shoots grown under a pressure of 2.3 atm., but the site of initiation is not critical for the subsequent growth of leaves in H. vulgaris . Lacunae in shoots grown under a pressure of 2.3 atm. were also larger than those in control shoots, implying that pressure does not constrict air spaces. Root growth is not inhibited by a pressure of 2.3 atm. Field measurements and laboratory experiments indicate that in warm water, PAR is the environmental factor most likely to control the depth limits of H. vulgaris in the field.     

The role of photosynthesis/light relationships in determining lower depth limits of Characeae in South Island, New Zealand lakes

1. The maximum depth of colonization of aquatic macrophytes ( Z _c) was investigated in eighteen South Island, New Zealand lakes. The downward attenuation coefficient for photosynthetically active radiation ( K _d(PAR)) was calculated and the spectral characteristics of the lakes determined with a spectroradiometer.
2. Characean algae dominated the deepest communities in sixteen of the study lakes.
3. Z _c was significantly related to K _d(PAR) by the relationship Z _c = 4.5/ K _d– 2.2.
4. From measurements of the photosynthetic properties of Chara corallina (Kl. ex Willd., em R.D.W.) and incident radiation over the course of a year we calculated the depth at which daily net photosynthesis would be equal to zero for each day of the year. An annual average of this depth was significantly related to Z _c with an r 2 of 0.86.
5. Correcting K _d(PAR) for spectral quality and taking into account the potential absorption spectrum of a characean meadow did not improve the relationships.
6. We suggest that relationships established between K _d(PAR) and Z _c of characean algae in South Island, New Zealand lakes can be explained to a great extent by light limitation of photosynthesis.     

The role of photosynthesis/light relationships in determining lower depth limits of Characeae in South Island, New Zealand lakes

1. The maximum depth of colonization of aquatic macrophytes ( Z _c) was investigated in eighteen South Island, New Zealand lakes. The downward attenuation coefficient for photosynthetically active radiation ( K _d(PAR)) was calculated and the spectral characteristics of the lakes determined with a spectroradiometer.
2. Characean algae dominated the deepest communities in sixteen of the study lakes.
3. Z _c was significantly related to K _d(PAR) by the relationship Z _c = 4.5/ K _d– 2.2.
4. From measurements of the photosynthetic properties of Chara corallina (Kl. ex Willd., em R.D.W.) and incident radiation over the course of a year we calculated the depth at which daily net photosynthesis would be equal to zero for each day of the year. An annual average of this depth was significantly related to Z _c with an r 2 of 0.86.
5. Correcting K _d(PAR) for spectral quality and taking into account the potential absorption spectrum of a characean meadow did not improve the relationships.
6. We suggest that relationships established between K _d(PAR) and Z _c of characean algae in South Island, New Zealand lakes can be explained to a great extent by light limitation of photosynthesis.     

Seagrass Herbivory Levels Sustain Site-Fidelity in a Remnant Dugong Population

Herds of dugong, a largely tropical marine megaherbivore, are known to undertake long-distance movements, sequentially overgrazing seagrass meadows in their path. Given their drastic declines in many regions, it is unclear whether at lower densities, their grazing is less intense, reducing their need to travel between meadows. We studied the effect of the feeding behaviour of a small dugong population in the Andaman and Nicobar archipelago, India to understand how small isolated populations graze seagrasses. In the seven years of our observation, all recorded dugongs travelled either solitarily or in pairs, and their use of seagrasses was limited to 8 meadows, some of which were persistently grazed. These meadows were relatively large, contiguous and dominated by short-lived seagrasses species. Dugongs consumed approximately 15% of meadow primary production, but there was a large variation (3–40% of total meadow production) in consumption patterns between meadows. The impact of herbivory was relatively high, with shoot densities c. 50% higher inside herbivore exclosures than in areas exposed to repeated grazing. Our results indicate that dugongs in the study area repeatedly graze the same meadows probably because the proportion of primary production consumed reduces shoot density to levels that are still above values that can trigger meadow abandonment. This ability of seagrasses to cope perhaps explains the long-term site fidelity shown by individual dugongs in these meadows. The fact that seagrass meadows in the archipelago are able to support dugong foraging requirements allows us to clearly identify locations where this remnant population persists, and where urgent management efforts can be directed.     

Seagrass landscapes: a terrestrial approach to the marine subtidal environment

Subtidal seagrass habitats are prime candidates for the application of principles derived from landscape ecology. Although seagrass systems are relatively simple compared to their terrestrial counterparts in terms of species diversity and structural complexity, seagrasses do display variation in spatial patterns over a variety of scales. The presence of a moving water layer and its influence on faunal dispersal may be a distinguishing feature impacting ecological processes in the subtidal zone. Studying seagrass-dominated landscapes may provide a novel approach to investigating questions regarding self-similarity of spatial patterns, and offers a new perspective for analysing habitat change in a variety of marine environments.     

Fast Detection of Nutrient Limitation in Macroalgae and Seagrass with Nutrient-Induced Fluorescence

Background

Rapid determination of which nutrients limit the primary production of macroalgae and seagrasses is vital for understanding the impacts of eutrophication on marine and freshwater ecosystems. However, current methods to assess nutrient limitation are often cumbersome and time consuming. For phytoplankton, a rapid method has been described based on short-term changes in chlorophyll fluorescence upon nutrient addition, also known as Nutrient-Induced Fluorescence Transients (NIFTs). Thus far, though, the NIFT technique was not well suited for macroalgae and seagrasses.

Methodology & Principal Findings

We developed a new experimental setup so that the NIFT technique can be used to assess nutrient limitation of benthic macroalgae and seagrasses. We first tested the applicability of the technique on sea lettuce (Ulva lactuca) cultured in the laboratory on nutrient-enriched medium without either nitrogen or phosphorus. Addition of the limiting nutrient resulted in a characteristic change in the fluorescence signal, whereas addition of non-limiting nutrients did not yield a response. Next, we applied the NIFT technique to field samples of the encrusting fan-leaf alga Lobophora variegata, one of the key algal species often involved in the degradation of coral reef ecosystems. The results pointed at co-limitation of L. variegata by phosphorus and nitrogen, although it responded more strongly to phosphate than to nitrate and ammonium addition. For turtle grass (Thalassia testudinum) we found the opposite result, with a stronger NIFT response to nitrate and ammonium than to phosphate.

Conclusions & Significance

Our extension of the NIFT technique offers an easy and fast method (30–60 min per sample) to determine nutrient limitation of macroalgae and seagrasses. We successfully applied this technique to macroalgae on coral reef ecosystems and to seagrass in a tropical inner bay, and foresee wider application to other aquatic plants, and to other marine and freshwater ecosystems.