Mangrove growth in New Zealand estuaries: the role of nutrient enrichment at sites with contrasting rates of sedimentation

Mangrove forest coverage is increasing in the estuaries of the North Island of New Zealand, causing changes in estuarine ecosystem structure and function. Sedimentation and associated nutrient enrichment have been proposed to be factors leading to increases in mangrove cover, but the relative importance of each of these factors is unknown. We conducted a fertilization study in estuaries with different sedimentation histories in order to determine the role of nutrient enrichment in stimulating mangrove growth and forest development. We expected that if mangroves were nutrient-limited, nutrient enrichment would lead to increases in mangrove growth and forest structure and that nutrient enrichment of trees in our site with low sedimentation would give rise to trees and sediments that converged in terms of functional characteristics on control sites in our high sedimentation site. The effects of fertilizing with nitrogen (N) varied among sites and across the intertidal zone, with enhancements in growth, photosynthetic carbon gain, N resorption prior to leaf senescence and the leaf area index of canopies being significantly greater at the high sedimentation sites than at the low sedimentation sites, and in landward dwarf trees compared to seaward fringing trees. Sediment respiration (CO₂ efflux) was higher at the high sedimentation site than at the low one sedimentation site, but it was not significantly affected by fertilization, suggesting that the high sedimentation site supported greater bacterial mineralization of sediment carbon. Nutrient enrichment of the coastal zone has a role in facilitating the expansion of mangroves in estuaries of the North Island of New Zealand, but this effect is secondary to that of sedimentation, which increases habitat area and stimulates growth. In estuaries with high sediment loads, enrichment with N will cause greater mangrove growth and further changes in ecosystem function.     

Salinity and light interactively affect neotropical mangrove seedlings at the leaf and whole plant levels

We have studied the interactive effects of salinity and light on Avicennia germinans mangrove seedlings in greenhouse and field experiments. We hypothesized that net photosynthesis, growth, and survivorship rates should increase more with an increase in light availability for plants growing at low salinity than for those growing at high salinity. This hypothesis was supported by our results for net photosynthesis and growth. Net daily photosynthesis did increase more with increasing light for low-salinity plants than for high-salinity plants. Stomatal conductance, leaf-level transpiration, and internal CO₂ concentrations were lower at high than at low salinity. At high light, the ratio of leaf respiration to assimilation was 2.5 times greater at high than at low salinity. Stomatal limitations and increased respiratory costs may explain why, at high salinity, seedlings did not respond to increased light availability with increased net photosynthesis. Seedling mass and growth rates increased more with increasing light availability at low than at high salinity. Ratios of root mass to leaf mass were higher at high salinity, suggesting that either water or nutrient limitations may have limited seedling growth at high salinity in response to increasing light. The interactive effects of salinity and light on seedling size and growth rates observed in the greenhouse were robust in the field, despite the presence of other factors in the field—such as inundation, nutrient gradients, and herbivory. In the field, seedling survivorship was higher at low than at high salinity and increased with light availability. Interestingly, the positive effect of light on seedling survivorship was stronger at high salinity, indicating that growth and survivorship rates are decoupled. In general, this study demonstrates that environmental effects at the leaf-level also influence whole plant growth in mangroves.     

Mangrove Seedling Net Photosynthesis,Growth, and Survivorship are Interactively Affected by Salinity and Light1

We hypothesized that salinity and light interactively affect mangroves, such that net photosynthesis, growth, and survivorship rates increase more with increase in light availability at low than high salinity. Using greenhouse and field experiments, we determined that net photosynthesis, growth rates, and size increased more with light at low than high salinity. At high salinity, the ratio of leaf respiration to assimilation increased fourfold, suggesting that salinity may have contributed to declines in net photosynthesis. Stomatal conductance, leaf‐level transpiration, and internal CO₂ concentrations were lower at high salinity. Ratios of root mass to leaf mass were higher at high salinity. Stomatal limitations and increased respiratory costs may explain why at high salinity, the seedlings did not respond to increased light availability with increased net photosynthesis. Increased root mass relative to leaf mass suggests that at high salinity, either water or nutrient limitations may have prevented the seedlings from increasing growth with increasing light availability. At both low‐ and high‐salinity zones in the field, seedling survivorship increased with light availability, and the effect of light was stronger at low salinity. However, at low light, survivorship was higher at high than low salinity, indicating that there may be a trade‐off between survivorship and growth. The interactive effects observed in the greenhouse were robust in the field, despite the presence of other factors in the field such as inundation and nutrient gradients and herbivory. This study provides a robust test of the hypothesis that salinity and light interactively effect mangrove seedling performance.     

Implication of nutrient and salinity interaction on the productivity of <Emphasis Type="Italic">Spartina patens</Emphasis>

Reintroduction of fresh water to coastal systems with altered hydrologic regimes is a management option for restoring degraded wetland habitats. Plant production in these systems is believed to be enhanced by increased nutrient availability and reduced salinity. Although studies have documented nutrient limitation and salinity stress in coastal marshes, interpreting the effects of freshwater reintroduction on plant production is difficult because high nutrient availability often is confounded with low salinity. We tested the hypothesis that plant growth response to nutrients does not vary with salinity in a greenhouse study. Treatments consisted of four nutrient concentrations and four non-lethal salinity levels; plant response was measured as biomass accumulation after 144 days of exposure. The significant interaction between salinity and nutrient concentrations indicates that response of Spartina patens marshes to freshwater inflows would vary by site-specific soil conditions. Biomass decreased with increased salinity at all four nutrient concentrations with variation among the nutrient concentrations decreasing as salinity increased. We demonstrate the importance of considering ambient salinity and nutrient soil conditions in restoration planning involving freshwater inflow. We propose salinity should remain a primary concern in restoration plans targeted at improving degraded S. patens-dominated marsh habitat.     

Germination and growth responses of hybridizing Carpobrotus species (Aizoaceae) from coastal California to soil salinity

Germination, growth, and physiological responses of hybridizing Carpobrotus from coastal California to soil salinity were studied. Hybrids are presumably the result of hybridization and introgression between the exotic Carpobrotus edulis, a succulent perennial invading coastal habitats, and the native or long-naturalized C. chilensis. Germination responses were investigated at 0, 10, 20, and 50% seawater. Seedling growth and physiology were compared by irrigating seedlings with solutions of the same seawater concentrations and in low and high nutrients. Germination was inhibited in the presence of salt, but recovered after transferring the seeds to fresh water. Seeds exposed to salt had higher final germination rates than control. Growth of Carpobrotus was slightly enhanced by low seawater concentrations but reduced at high salinity at both nutrient regimes. Leaf cell sap osmolarity increased with increasing soil salinity, and taxa did not differ significantly in this physiological adjustment. Leaf carbon isotope ratios (∂¹³C) ranged from −28 to −22‰ and became less negative at higher salinities, indicating an improved water use efficiency in the seedlings at high salt concentrations. In addition, ∂¹³C values were generally less negative at high than at low nutrients. Differences among taxa were generally small. The results show that salinity affects both establishment and growth of hybridizing Carpobrotus. The overall weak species differences in salt tolerance indicate that the exotic C. edulis can occupy the same sites as C. chilensis in terms of salinity. The similarity of hybrids in their response to salinity suggests that they may contribute to the invasion by Carpobrotus.     

The effect of atmospheric carbon dioxide concentrations on the performance of the mangrove Avicennia germinans over a range of salinities

By increasing water use efficiency and carbon assimilation, increasing atmospheric CO₂ concentrations could potentially improve plant productivity and growth at high salinities. To assess the effect of elevated CO₂ on the salinity response of a woody halophyte, we grew seedlings of the mangrove Avicennia germinans under a combination of five salinity treatments [from 5 to 65 parts per thousand (ppt)] and three CO₂ concentrations (280, 400 and 800 ppm). We measured survivorship, growth rate, photosynthetic gas exchange, root architecture and foliar nutrient and ion concentrations. The salinity optima for growth shifted higher with increasing concentrations of CO₂, from 0 ppt at 280 ppm to 35 ppt at 800 ppm. At optimal salinity conditions, carbon assimilation rates were significantly higher under elevated CO₂ concentrations. However, at salinities above the salinity optima, salinity had an expected negative effect on mangrove growth and carbon assimilation, which was not alleviated by elevated CO₂, despite a significant improvement in photosynthetic water use efficiency. This is likely due to non‐stomatal limitations to growth at high salinities, as indicated by our measurements of foliar ion concentrations that show a displacement of K⁺ by Na⁺ at elevated salinities that is not affected by CO₂. The observed shift in the optimal salinity for growth with increasing CO₂ concentrations changes the fundamental niche of this species and could have significant effects on future mangrove distribution patterns and interspecific interactions.     

Hydrodynamics of mangrove swamps and their coastal waters

Mangrove swamps help control the tidal hydrodynamics of many tropical estuaries. They generate an asymmetry of the tidal currents in both the tidal creeks and the mangrove swamps. This results in self-scouring of the tidal channels. Mangrove land reclamation results in siltation of the channel. Mangrove swamps control the flushing rates of the estuaries through the lateral trapping effect. Lateral trapping leads to the aggregation of mangrove litter along slick lines. Evapotranspiration plays a role in the hot dry season by forming a salinity maximum zone which isolates the estuary from the coastal waters for several months of the year. In the absence of runoff, evapotranspiration in the hot dry season generates an inverse estuarine circulation which can trap high salinity mangrove water, and mangrove detritus, along the bottom of a mangrove creek. This bottom layer can become anaerobic. Groundwater flow appears to play a key role in the nutrient budget of mangrove creeks, exporting salt left behind by evapotranspiration, and inhibiting runoff after rainfall. Particulates and dissolved nutrients outwelled from mangrove swamps to coastal waters are retained in a coastal boundary layer. This coastal boundary layer water can be trapped along the shore for long periods if the coast is straight and mangrove-fringed and the coastal waters are shallow. Headlands inhibit coastal trapping because they enhance mixing. Nutrient-rich coastal boundary layer waters may be ejected offshore as tidal jets peeling off headlands and locally enriching offshore waters.     

Growth and water use of the mangroves Rhizophora apiculata and R. stylosa in response to salinity and humidity under ambient and elevated concentrations of atmospheric CO2

Two mangrove species, Rhizophora apiculata and R. stylosa, were grown for 14 weeks in a multifactorial combination of salinity (125 and 350 mol m^?3 NaCl), humidity (43 and 86% relative humidity at 30°C) and atmospheric CO₂ concentration (340 and 700 cm³ m^?3). Under ambient [CO₂], growth responses to different combinations of salinity and humidity were consistent with interspecific differences in distribution along natural gradients of salinity and aridity in northern Australia. Elevated [CO₂] had little effect on relative growth rate when it was limited by salinity but stimulated growth when limited by humidity. Both species benefited most from elevated [CO₂] under relatively low salinity conditions in which growth was vigorous, but relative growth rate was enhanced more in the less salt-tolerant and more rapidly growing species, R. apiculata. Changes in both net assimilation rate and leaf area ratio contributed to changes in relative growth rates under elevated [CO₂], with leaf area ratio increasing with decrease in humidity. Increase in water use efficiency under elevated [CO₂] occurred with increase, decrease or no change in evaporation rates; water use characteristics which depended on both the species and the growth conditions. In summary, elevated [CO₂] is unlikely to increase salt tolerance, but could alter competitive rankings of species along salinity × aridity gradients.     

Variable effects of nutrient enrichment on soil respiration in mangrove forests

Background and Aims

Mangrove forests are globally important sites of carbon burial that are increasingly exposed to nutrient pollution. Here we assessed the response of soil respiration, an important component of forest carbon budgets, to nutrient enrichment over a wide range of mangrove forests.

Methods

We assessed the response of soil respiration to nutrient enrichment using fertilization experiments within 22 mangrove forests over ten sites. We used boosted regression tree (BRT) models to determine the importance of environmental and plant factors for soil respiration and its responsiveness to fertilizer treatments.

Results

Leaf area index explained the largest proportion of variation in soil respiration rates (LAI, 45.9 %) followed by those of site, which had a relative influence of 39.9 % in the BRT model. Nutrient enrichment enhanced soil respiration only in nine out of 22 forests. Soil respiration in scrub forests showed a positive response to nutrient addition more frequently than taller fringing forests. The response of soil respiration to nutrient enrichment varied with changes in specific leaf area (SLA) and stem extension, with relative influences of 14.4 %, 13.6 % in the BRT model respectively.

Conclusions

Soil respiration in mangroves varied with LAI, but other site specific factors also influenced soil respiration and its response to nutrient enrichment. Strong enhancements in aboveground growth but moderate increases in soil respiration with nutrient enrichment indicated that nutrient enrichment of mangrove forests has likely increased net ecosystem production.     

Carbon and nutrient exchange of mangrove forests with the coastal ocean

Mangrove forests exchange materials with the coastal ocean through tidal inundation. In this study, we aim to provide an overview of the published data of carbon (C) and nutrient exchange of mangrove forests with the coastal ocean at different spatial scales to assess whether the exchange is correlated with environmental parameters. We collected data on C (dissolved and particulate organic C; DOC and POC) and nutrient exchange (dissolved and particulate nitrogen, N and phosphorus, P) and examined the role of latitude, temperature, precipitation, geomorphological setting, hydrology, dominant mangrove species and forest area in explaining the variability of the exchange. We identified that there are a range of methodologies used to determine material exchange of mangroves with the coastal zone, each methodology providing data on the exchange at different spatial scales. This variability of approaches has limited our understanding of the role of mangroves in the coastal zone. Regardless, we found that mangrove forests export C and nutrients to the coastal zone in the form of litter and POC. We found that precipitation is a major factor influencing the export of C in the form of litter; sites with low annual precipitation and high mean annual temperatures export more C as litter than sites with high precipitation and low temperature. Furthermore, export of POC is higher in zones with low mean annual minimum temperature. Identification of broad-scale trends in DOC and dissolved nutrients was more difficult, as the analysis was limited by scarcity of suitable studies and high variability in experimental approaches. However, tidal amplitude and the concentration of nutrients in the floodwater appears to be important in determining nutrient exchange. The strongest conclusion from our analysis is that mangrove forests are in general sources of C and nutrients in the form of litter and POC and that they are most likely to be exporting C subsidies in dry regions.     

Variation in nutrient limitation and seagrass nutrient content in Bahamian tidal creek ecosystems

The traditional model of nutrient availability in coastal estuarine ecosystems is based on predictable inputs of nitrogen (N) and phosphorus (P) via riverine and oceanic sources, respectively. But coastlines with low nutrient input from these sources may not fit into this simple framework. Here we use observational (seagrass nutrient content) and experimental (nutrient enrichment assays) data for assessing nutrient availability and limitation for primary producers along a spatial transect extending from the mouth (nearest to the ocean) to the terminal portion (boundary with the terrestrial ecosystem) of three coastal mangrove-lined tidal creeks in The Bahamas. Compiling seagrass nutrient content from all sites showed a negative relationship between seagrass nutrient limitation (either N or P) and distance from mouth, but this pattern differed across sites with respect to which nutrient was more limiting. Our experimental results demonstrated patterns of decreased response by microalgae to dual nutrient enrichment in one site with distance from the creek mouth, and increased response to single nutrient enrichment in another, with the third showing no trend along this gradient. Our findings show that Bahamian mangrove wetlands are extremely nutrient-limited ecosystems, and that the most limiting nutrient varied among sites. In general, these ecosystems deviate from the typical paradigm of spatial nutrient limitation patterns in estuaries. We suggest that various site-specific biological and physical factors may be more important than large-scale hydrologic factors in driving trends of nutrient availability in coastal ecosystems under strong nutrient constraints, such as in The Bahamas. Our findings suggest that even minor changes in nutrient loading rates can have significant implications for primary production in subtropical oligotrophic systems.     

Bryozoan populations reflect nutrient enrichment and productivity gradients in rivers

1. The hypothesis that nutrient enrichment will affect bryozoan abundance was tested using two complementary investigations; a field‐based method determining bryozoan abundance in 20 rivers of different nutrient concentrations by deploying statoblast (dormant propagule) traps and an experimental laboratory microcosm study measuring bryozoan growth and mortality. These two methods confirmed independently that increased nutrient concentrations in water promote increases in the biomass of freshwater bryozoans. 2. Statoblasts of the genus Plumatella were recorded in all rivers, regardless of nutrient concentrations, demonstrating that freshwater bryozoans are widespread. Concentrations of Plumatella statoblasts were high in rivers with high nutrient concentrations relative to those with low to moderate nutrient concentrations. Regression analyses indicated that phosphorus concentrations, in particular, significantly influenced statoblast concentrations. 3. Concentrations of Lophopus crystallinus statoblasts were also higher in sites characterised by high nutrient concentrations. Logistic regression analysis revealed that the presence of L. crystallinus statoblasts was significantly associated with decreasing altitude and increasing phosphorus concentrations. This apparently rare species was found in nine rivers (out of 20), seven of which were new sites for L. crystallinus. 4. Growth rates of Fredericella sultana in laboratory microcosms increased with increasing nutrient concentration and high mortality rates were associated with low nutrient concentrations. 5. Our results indicate that bryozoans respond to increasing nutrient concentrations by increased growth, resulting in higher biomasses in enriched waters. We also found that an important component of bryozoan diets can derive from food items lacking chlorophyll a. Finally, bryozoans may be used as independent proxies for inferring trophic conditions, a feature that may be especially valuable in reconstructing historical environments by assessing the abundance of statoblasts in sediment cores.     

Eutrophication and macroalgal blooms in temperate and tropical coastal waters: nutrient enrichment experiments with Ulva spp.

Receiving coastal waters and estuaries are among the most nutrient‐enriched environments on earth, and one of the symptoms of the resulting eutrophication is the proliferation of opportunistic, fast‐growing marine seaweeds. Here, we used a widespread macroalga often involved in blooms, Ulva spp., to investigate how supply of nitrogen (N) and phosphorus (P), the two main potential growth‐limiting nutrients, influence macroalgal growth in temperate and tropical coastal waters ranging from low‐ to high‐nutrient supplies. We carried out N and P enrichment field experiments on Ulva spp. in seven coastal systems, with one of these systems represented by three different subestuaries, for a total of nine sites. We showed that rate of growth of Ulva spp. was directly correlated to annual dissolved inorganic nitrogen (DIN) concentrations, where growth increased with increasing DIN concentration. Internal N pools of macroalgal fronds were also linked to increased DIN supply, and algal growth rates were tightly coupled to these internal N pools. The increases in DIN appeared to be related to greater inputs of wastewater to these coastal waters as indicated by high δ¹⁵N signatures of the algae as DIN increased. N and P enrichment experiments showed that rate of macroalgal growth was controlled by supply of DIN where ambient DIN concentrations were low, and by P where DIN concentrations were higher, regardless of latitude or geographic setting. These results suggest that understanding the basis for macroalgal blooms, and management of these harmful phenomena, will require information as to nutrient sources, and actions to reduce supply of N and P in coastal waters concerned.     

Effects of Salinity Fluctuation on Photosynthetic Gas Exchange and Plant Growth of The Red Mangrove (Rhizophora mangle L.)

The red mangrove (Rhizophora mangle L.) in southern Floridaoccurs frequently in two distinct growth forms, tall and scrubplants, with the scrub form usually found in coastal inlandareas having a higher fluctuation of soil water salinity. Inthe present study, effects of constant and fluctuating salinitieson leaf gas exchange and plant growth of red mangrove seedlingswere investigated under greenhouse conditions. Both constantand fluctuating salinity treatments significantly affected leafgas exchange and plant growth of red mangrove seedlings. Seedlingssubjected to the fluctuating salinity with the mean of both100 and 250 mol m^–3 NaCl showed significantly lower photosynthesisand plant growth than those subjected to the corresponding constantsalinity with the same mean. The photosynthetic and growth ratesof the seedlings under these fluctuating treatments were aslow as, or even lower than those expected if they were growingunder the high constant salinity of their respective fluctuationtreatments. Seedlings subjected to the fluctuating salinitywith the mean of 500 mol m^–3 NaCl, however, demonstratedslightly higher CO₂ assimilation rate and stomatal conductance,but the same plant growth rates as those under the constant500 mol m^–3 NaCl treatment. These results suggest that,in general, fluctuating salinity has significant negative effectson photosynthesis and plant growth relative to constant salinitywith the same mean. If this finding can be applicable to fieldsituations, the low photosynthesis and plant growth observedpreviously in several scrub mangrove forests probably can beattributed in part to the salinity fluctuation of soil waterin these mangrove forests. Key words: Fluctuating salinity, photosynthesis, growth, growth forms, mangroves     

The Nutrient Status of Mgazana,a Warm Temperate Mangrove Estuary in the Transkei,Eastern Cape,South Africa

Mgazana, a rural southern African mangrove system, was visited monthly from August, 1995 to February, 1997 to collect water samples for nutrient analysis. Surface and bottom samples were taken during spring low tide at seven stations along the estuary and the following physico-chemical parameters measured: river flow, temperature, salinity, oxygen, transparency, ammonia, nitrite, nitrate, phosphate, inorganic carbon (IC), organic carbon (OC), total carbon (TC), soluble nitrogen (SN), particulate nitrogen (PN) and total nitrogen (TN). Using correlation matrix analysis and ANOVA, river flow was found to affect estuarine salinity, transparency and stratification, which influenced nutrient dynamics. Significant seasonal (winter and summer) differences were found for temperature, river flow, nitrate, SN, TN, IC and OC. Most nutrients were significantly correlated with river flow showing gradients down the estuary, indicating allochthonous input from the catchment. OC levels within the estuary were high, probably due to autochthonous mangrove leaf-fall processing by the various in-fauna, but high levels measured at the head of the estuary during high river flow suggested additional allochthonous input from coastal forest litter. Conversely, IC was negatively correlated with river flow suggesting that autochthonous faunal and microbial mineralisation of organic matter occurs within creeks, which is then diluted by increased stream-flow. An N:P ratio of 2.7:1 was obtained for this rural mangrove system, which was low compared with Spartina-based East Cape estuaries subject to urban, industrial and agricultural pollution.     

Intra- and interspecific facilitation in mangroves may increase resilience to climate change threats

Mangroves are intertidal ecosystems that are particularly vulnerable to climate change. At the low tidal limits of their range, they face swamping by rising sea levels; at the high tidal limits, they face increasing stress from desiccation and high salinity. Facilitation theory may help guide mangrove management and restoration in the face of these threats by suggesting how and when positive intra- and interspecific effects may occur: such effects are predicted in stressed environments such as the intertidal, but have yet to be shown among mangroves. Here, we report the results of a series of experiments at low and high tidal sites examining the effects of mangrove density and species mix on seedling survival and recruitment, and on the ability of mangroves to trap sediment and cause surface elevation change. Increasing density significantly increased the survival of seedlings of two different species at both high and low tidal sites, and enhanced sediment accretion and elevation at the low tidal site. Including Avicennia marina in species mixes enhanced total biomass at a degraded high tidal site. Increasing biomass led to changed microenvironments that allowed the recruitment and survival of different mangrove species, particularly Ceriops tagal.     

Application of stable isotopes to examine N proportions within a simulated Aegiceras corniculatum wetland

Salinity levels and drought status of coastal wetlands may be strongly affected by climate change, and changes in the nitrogen cycle of mangrove wetlands may also be affected. We established combinations of three salinity and water levels with applied stable isotope ¹⁵N to study the δ¹⁵N distributions in the sediment and plants of a greenhouse-based simulated mangrove Aegiceras corniculatum wetland system. The stable isotope ¹³C and ¹⁵N, C and N contents and the C:N ratio were determined. Results showed that increasing in salinity significantly increased the δ¹³C value in plant organs. The δ¹⁵N value of plant organs increased with increasing water level in low salinity (10‰) and medium salinity (20‰) treatment groups but not in the high salinity (30‰) treatment group. This may attributed to A. corniculatum adjusting the δ¹⁵N distribution in different organs in response to high salinity stress. Compared to the δ¹³C, the δ¹⁵N values of plant were strongly affected by salinity and water level treatments, indicating that the behavior of N cycle was somewhat different than the C cycle, and affected by the combined effects of both salinity and water level. Most of ¹⁵N absorbed by plant tissues were in leaves except for the highest salinity and high water level treatment, showing at increasing water level, the proportion of ¹⁵N increased in root. Overall, the measured indicators exhibited different responses to salinity level and water level, suggesting that the changes in salinity and water levels have an impact on N cycling processes of wetland systems.     

Environmental drivers in mangrove establishment and early development: A review

Mangroves have a global distribution within coastal tropical and subtropical climates, and have even expanded to some temperate locales. Where they do occur, mangroves provide a plethora of goods and services, ranging from coastal protection from storms and erosion to direct income for human societies. The mangrove literature has become rather voluminous, prompting many subdisciplines within a field that earlier in the 20th century received little focus. Much of this research has become diffuse by sheer numbers, requiring detailed syntheses to make research results widely available to resource managers. In this review, we take an inclusive approach in focusing on eco-physiological and growth constraints to the establishment and early development of mangrove seedlings in the intertidal zone. This is a critical life stage for mangroves, i.e., the period between dispersal and recruitment to the sapling stage. We begin with some of the research that has set the precedent for seedling-level eco-physiological research in mangroves, and then we focus on recent advances (circa. 1995 to present) in our understanding of temperature, carbon dioxide, salinity, light, nutrient, flooding, and specific biotic influences on seedling survival and growth. As such, we take a new approach in describing seedling response to global factors (e.g., temperature) along with site-specific factors (e.g., salinity). All variables will strongly influence the future of seedling dynamics in ways perhaps not yet documented in mature forests. Furthermore, understanding how different mangrove species can respond to global factors and regional influences is useful for diagnosing observed mortality within mangrove wetlands, managed or natural. This review provides an updated eco-physiological knowledge base for future research and reforestation activity, and for understanding important links among climate change, local physico-chemical condition, and establishment and early growth of mangrove seedlings.     

Effects of various mixed salt-alkaline stresses on growth,photosynthesis, and photosynthetic pigment concentrations of <Emphasis Type="Italic">Medicago ruthenica</Emphasis> seedlings

Soil salinization and alkalinization frequently co-occur in naturally saline and alkaline soils. To understand the characteristics of mixed salt-alkali stress and adaptive response of Medicago ruthenica seedlings to salt-alkali stress, water content of shoots, growth and photosynthetic characteristics of seedlings under 30 salt-alkaline combinations (salinity 24–120 mM and pH 7.03–10.32) with mixed salts (NaCl, Na₂SO₄, NaHCO₃, and Na₂CO₃) were examined. The indices were significantly affected by both salinity and pH. The interactive effects between salt and alkali stresses were significant, except for photosynthetic pigments. Water content of shoots, relative growth rates of shoots and roots and pigment concentrations showed decreasing trends with increasing salinity and alkalinity. The root activity under high alkalinity and salinity treatments gradually decreased, but was stimulated by the combined effects of low alkalinity and salinity. The survival rate decreased with increased salinity, except at pH 7.03–7.26 when all plants survived. Net photosynthetic rate, stomatal conductance and intercellular CO₂ concentration decreased with increased salinity and pH. M. ruthenica tolerated the stress of high salt concentration when alkali concentration was low, and the synergistic effects of high alkali and high salt concentrations lead to the death of some or all seedlings. M. ruthenica appeared to be saltalkali tolerant. Reducing the salt concentration or pH based on the salt components in the soil may be helpful to abate damage from mixed salt-alkaline stress.     

Identifying the interacting roles of stressors in driving the global loss of canopy‐forming to mat‐forming algae in marine ecosystems

Identifying the type and strength of interactions between local anthropogenic and other stressors can help to set achievable management targets for degraded marine ecosystems and support their resilience by identifying local actions. We undertook a meta‐analysis, using data from 118 studies to test the hypothesis that ongoing global declines in the dominant habitat along temperate rocky coastlines, forests of canopy‐forming algae and/or their replacement by mat‐forming algae are driven by the nonadditive interactions between local anthropogenic stressors that can be addressed through management actions (fishing, heavy metal pollution, nutrient enrichment and high sediment loads) and other stressors (presence of competitors or grazers, removal of canopy algae, limiting or excessive light, low or high salinity, increasing temperature, high wave exposure and high UV or CO₂), not as easily amenable to management actions. In general, the cumulative effects of local anthropogenic and other stressors had negative effects on the growth and survival of canopy‐forming algae. Conversely, the growth or survival of mat‐forming algae was either unaffected or significantly enhanced by the same pairs of stressors. Contrary to our predictions, the majority of interactions between stressors were additive. There were however synergistic interactions between nutrient enrichment and heavy metals, the presence of competitors, low light and increasing temperature, leading to amplified negative effects on canopy‐forming algae. There were also synergistic interactions between nutrient enrichment and increasing CO₂ and temperature leading to amplified positive effects on mat‐forming algae. Our review of the current literature shows that management of nutrient levels, rather than fishing, heavy metal pollution or high sediment loads, would provide the greatest opportunity for preventing the shift from canopy to mat‐forming algae, particularly in enclosed bays or estuaries because of the higher prevalence of synergistic interactions between nutrient enrichment with other local and global stressors, and as such it should be prioritized.