Nitrogen versus phosphorus limitation in a subtropical coastal embayment (Moreton Bay; Australia): Implications for management

Nitrogen loading resulting from major floods and sediment resuspension to a large coastal embayment

Major floods pose a severe threat to coastal receiving environments, negatively impacting environmental health and ecosystem services through direct smothering with sediment and nutrient loading. This study examined the short and long-term impacts of the February 2022 major flood event on mud extent and sediment nitrogen flux in Moreton Bay (the Bay), a large, sub-tropical embayment in Southeast Queensland, Australia. Short-term impacts were assessed three days after the flood peak by sampling surface water at 47 sites in the direction of the predominant circulation pattern. Longer-term impacts were assessed by undertaking an intensive sediment survey of 223 sites and a nutrient flux experiment using sediment core incubations to simulate calm and resuspension conditions for the four key sediment classes. Short-term impacts revealed elevated turbidity levels extended across the Bay but were highest at the Brisbane River mouth, ammonium concentrations varied inversely with surface turbidity, whereas nitrate concentrates closely tracked surface turbidity. The sediment survey confirmed fine sediment deposition across 98 % of the Bay. Porewater within the upper 10 cm contained a standing pool of 280 t of ammonium, with concentrations more than three orders of magnitude higher than overlying surface waters. The nutrient flux experiment revealed an order of magnitude higher sediment ammonium flux rate in the sandy mud sediment class compared to the other sediment classes; and for simulated resuspension conditions compared to calm conditions for sand, muddy sand, and mud sediment classes. Scaling across the whole Bay, we estimated a mean annual sediment flux of 17,700 t/year ammonium, with a range of 13,500 to 21,900 t/year. Delivery of fine sediments by major floods over the last 50 years now impact >98 % of the benthic zone and provide a major loading pathway of available nitrogen to surface waters of Moreton Bay; representing a significant threat to ecosystem health.

Urban stormwater characterisation and nitrogen composition from lot-scale catchments - New management implications

Stormwater runoff from urban areas has been shown to contain a variety of pollutants which are often linked to the specific land use of the catchment. This research program investigated the pollutant concentrations in stormwater runoff from several sites in South-east Queensland (SEQ), in Australia. The study sites are predominantly single development lots, under 7.5hectares (Ha) in area, with a single land-use classification that have been developed with stormwater treatment measures to manage pollutant loads as required by local regulations. The testing program also analysed the nitrogen composition in the catchment runoff samples (prior to treatment) during storm events and compared them to current Australian guidelines. The results to date (n=320) have shown pollutant concentrations to be significantly lower than those historically published as typical for Australian land uses (p<0.05). Ongoing application of out-dated influent values as part of development assessment processes could potentially provide inaccurate results, incorrectly sized and under-performing treatment measures. This current research also suggests that nitrogen in runoff from lot-scale, urban residential catchments has average nitrogen oxides (NOx) ~16% and ammonia ~9% as percentage of total nitrogen (TN). Total Kjeldahl nitrogen (TKN) forms on average ~84% of the total nitrogen concentration during events. Where it was previously recommended that to achieve water quality targets of 45% total nitrogen load reduction, treatment measures targeting NOx were required (e.g. Vegetated systems), this latest research indicates that solutions removing organic nitrogen also may be necessary, increasing the options available to designers.

Attributes of successful actions to restore lakes and estuaries degraded by nutrient pollution

As more success is achieved in restoring lakes and estuaries from the impacts of nutrient pollution, there is increased opportunity to evaluate the scientific, social, and policy factors associated with achieving restoration goals. We examined case studies where deliberate actions to reduce nutrient pollution and restore ecosystems resulted in ecological recovery. Prospective cases were identified from scientific literature and technical documents for lakes and estuaries with: (1) scientific evidence of nutrient pollution; (2) restoration actions taken to mitigate nutrient pollution; and (3) documented ecological improvement. Using these criteria, we identified 9 estuaries and 7 lakes spanning countries, climatic regions, physical types, depths, and watershed areas. Among 16 case studies ultimately included, 8 achieved improvements short of stated restoration goals. Five more were successful initially, but condition subsequently declined. Three of the case studies achieved their goals fully and are currently managing to maintain the restored condition. We examined each case to identify both common attributes of nutrient management, grouped into 'themes', and variations on those attributes, which were coded into categorical variables based on thorough review of documents associated with each case. The themes and variables were organized into a broad conceptual model illustrating how they relate to each other and to nutrient management outcomes. We then explored relationships among the themes and variables using multiple correspondence analysis (MCA). Results of the MCA suggested that the attributes most associated with achieving restoration goals include: (1) leadership by a dedicated watershed management agency; (2) governance through a bottom-up collaborative process; (3) a strategy that set numeric targets based on a specific ecological goal; and (4) actions to reduce nutrient loads from all sources. While our study did not provide a comprehensive road map to successful nutrient management, it suggested attributes that could be emulated in future efforts. The quantitative approach that was applied could be used to provide ongoing analysis as new examples of nutrient management success emerge.

Succession of phytoplankton assemblages in response to large-scale reservoir operation: a case study in a tributary of the Three Gorges Reservoir, China

The Three Gorges Dam (TGD) has greatly altered ecological and environmental conditions within the reservoir region, but it is not known how these changes affect phytoplankton structure and dynamics. Here, a bimonthly monitoring program was implemented from 2007 to 2009 to study the impact of damming on phytoplankton assemblages in the backwater area of the Pengxi River (PBA). By application of the phytoplankton functional group (C strategists, competitive species; S strategists, stress-tolerant species; R strategists, rapid propagation species), seasonal changes in phytoplankton relative to environmental variations were evaluated using ordination analysis. Seasonal patterns of phytoplankton dynamics were detected during this study, with CS/S strategists causing algal blooms from mid-spring to early summer, CS/CR strategists often observed during flood season, and CS strategists dominant during mid-autumn. CR/R groups dominated during winter and caused algal blooms in February. Our results indicated that phytoplankton assemblages were directly related to reservoir operation effects. Generally, the TGD had a low water level during flood season, resulting in a relatively short hydraulic retention time and intensive variability, which supported the cooccurrence of CS and CR species. During the winter drought season, water storage in the TGD increased the water level and the hydraulic retention time in the PBA, enabling R/CR strategists to overcome the sedimentation effect and to out-compete S/CS species in winter. As expected, these diversity patterns were significantly correlated with the hydraulic retention time and nutrient limitation pattern in the PBA. This study provides strategic insight for evaluating the impacts of reservoir operations on phytoplankton adaptation.

Nitrogen sharing and water source partitioning co-occur in estuarine wetlands

Plant-plant interactions are particularly complex in multi-resource limited environments. The aim of this study was to assess species interactions in estuarine wetlands where both N and fresh water are limited. We combined stable isotope methods and dissimilarity analyses to compare interspecific interactions in N source use and water source use. Both Melaleuca quinquenervia (Cav.) S. T Blake and Avicennia marina (Forssk.) Vierh. had a lower leaf δ15N when they were growing together with the N-fixer Casuarina glauca Sieb. ex Spreng. compared with those trees growing in monospecific stands, but their water isotopes, δ18O and δD, were different from C. glauca. Our results indicate that the N-fixer C. glauca shared their N with co-existing neighbours, either indirectly or directly, but that water sources were partitioned among them. Further analyses showed that M. quinquenervia and C. glauca had lower dissimilarity in N source use but higher dissimilarity in water source use than the C. glauca-A. marina pair, implying that the co-existence between M. quinquenervia and C. glauca is relatively stable. Our results suggest that facilitative interaction and resource partitioning can co-occur in estuarine wetlands, and which could be important in maintaining diversity across resource gradients.

Evidence of large-scale chronic eutrophication in the Great Barrier Reef: quantification of chlorophyll a thresholds for sustaining coral reef communities

Long-term monitoring data show that hard coral cover on the Great Barrier Reef (GBR) has reduced by >70 % over the past century. Although authorities and many marine scientists were in denial for many years, it is now widely accepted that this reduction is largely attributable to the chronic state of eutrophication that exists throughout most of the GBR. Some reefs in the far northern GBR where the annual mean chlorophyll a (Chl a) is in the lower range of the proposed Eutrophication Threshold Concentration for Chl a (~0.2-0.3 mg m⁻³) show little or no evidence of degradation over the past century. However, the available evidence suggests that coral diseases and the crown-of-thorns starfish will proliferate in such waters and hence the mandated eutrophication Trigger values for Chl a (~0.4-0.45 mg m⁻³) will need to be decreased to ~0.2 mg m⁻³ for sustaining coral reef communities.

Proteomic and transcriptomic analyses of "Candidatus Pelagibacter ubique" describe the first PII-independent response to nitrogen limitation in a free-living Alphaproteobacterium

Nitrogen is one of the major nutrients limiting microbial productivity in the ocean, and as a result, most marine microorganisms have evolved systems for responding to nitrogen stress. The highly abundant alphaproteobacterium "Candidatus Pelagibacter ubique," a cultured member of the order Pelagibacterales (SAR11), lacks the canonical GlnB, GlnD, GlnK, and NtrB/NtrC genes for regulating nitrogen assimilation, raising questions about how these organisms respond to nitrogen limitation. A survey of 266 Alphaproteobacteria genomes found these five regulatory genes nearly universally conserved, absent only in intracellular parasites and members of the order Pelagibacterales, including "Ca. Pelagibacter ubique." Global differences in mRNA and protein expression between nitrogen-limited and nitrogen-replete cultures were measured to identify nitrogen stress responses in "Ca. Pelagibacter ubique" strain HTCC1062. Transporters for ammonium (AmtB), taurine (TauA), amino acids (YhdW), and opines (OccT) were all elevated in nitrogen-limited cells, indicating that they devote increased resources to the assimilation of nitrogenous organic compounds. Enzymes for assimilating amine into glutamine (GlnA), glutamate (GltBD), and glycine (AspC) were similarly upregulated. Differential regulation of the transcriptional regulator NtrX in the two-component signaling system NtrY/NtrX was also observed, implicating it in control of the nitrogen starvation response. Comparisons of the transcriptome and proteome supported previous observations of uncoupling between transcription and translation in nutrient-deprived "Ca. Pelagibacter ubique" cells. Overall, these data reveal a streamlined, PII-independent response to nitrogen stress in "Ca. Pelagibacter ubique," and likely other Pelagibacterales, and show that they respond to nitrogen stress by allocating more resources to the assimilation of nitrogen-rich organic compounds.

IMPORTANCE

Pelagibacterales are extraordinarily abundant and play a pivotal role in marine geochemical cycles, as one of the major recyclers of labile dissolved organic matter. They are also models for understanding how streamlining selection can reshape chemoheterotroph metabolism. Streamlining and its broad importance to environmental microbiology are emerging slowly from studies that reveal the complete genomes of uncultured organisms. Here, we report another remarkable example of streamlined metabolism in Pelagibacterales, this time in systems that control nitrogen assimilation. Pelagibacterales are major contributors to metatranscriptomes and metaproteomes from ocean systems, where patterns of gene expression are used to gain insight into ocean conditions and geochemical cycles. The data presented here supply background that is essential to interpreting data from field studies.

Changes to chronic nitrogen loading from sewage discharges modify standing stocks of coastal phytoplankton

Nutrient delivery in subtropical coastal systems is predominantly via acute episodic high flow events. However, continuous nutrient discharges from point sources alter these natural fluctuations in nutrient delivery, and are therefore likely to lead to different ecosystem responses. The aim of this study was to assess how a reduction in chronic sewage nutrient inputs affected chlorophyll a (chl a) concentrations in a subtropical bay, in the context of seasonal fluctuations in riverine nutrient inflows. Reduced nutrient inputs from a large sewage treatment plant (STP) resulted in lower mean dissolved inorganic nitrogen and phytoplankton chl a concentrations during both the austral summer wet and winter dry season. This was measurable within 10 y of nutrient reductions and despite the confounding effects of nutrient inflow events. Our study demonstrates that reductions in STP inputs can have significant effects on phytoplankton biomass despite confounding factors over relatively short time frames.

High congruence of isotope sewage signals in multiple marine taxa

Assessments of sewage pollution routinely employ stable nitrogen isotope analysis (δ(15)N) in biota, but multiple taxa are rarely used. This single species focus leads to underreporting of whether derived spatial N patterns are consistent. Here we test the question of 'reproducibility', incorporating 'taxonomic replication' in the measurement of δ(15)N gradients in algae, seagrasses, crabs and fish with distance from a sewage outfall on the Adelaide coast (southern Australia). Isotopic sewage signals were equally strong in all taxa and declined at the same rate. This congruence amongst taxa has not been reported previously. It implies that sewage-N propagates to fish via a tight spatial coupling between production and consumption processes, resulting from limited animal movement that closely preserves the spatial pollution imprint. In situations such as this where consumers mirror pollution signals of primary producers, analyses of higher trophic levels will capture a broader ambit of ecological effects.