A primary focus of coastal science during the past 3 decades has been the question: How does anthropogenic nutrient enrichment cause change in the structure or function of nearshore coastal ecosystems? This theme of environmental science is recent, so our conceptual model of the coastal eutrophication problem continues to change rapidly In this review, I suggest that the early (Phase I) con- ceptual model was strongly influenced by limnologists, who began intense study of lake eutrophication by the 1960s The Phase I model emphasized changing nutrient input as a signal, and responses to that signal as increased phytoplankton biomass and primary production, decomposition of phytoplankton- derived organic matter, and enhanced depletion of oxygen from bottom waters Coastal research in recent decades has identified key differences in the responses of lakes and coastal-estuarine ecosystems to nutrient enrichment The contemporary (Phase II) conceptual model reflects those differences and includes explicit recognition of (1) system-specific attributes that act as a filter to modulate the responses to enrichment (leading to large differences among estuarine-coastal systems in their sensitivity to nu- trient enrichment); and (2) a complex suite of direct and indirect responses including linked changes in: water transparency, distribution of vascular plants and biomass of macroalgae, sediment biogeochem- istry and nutrient cycling, nutrient ratios and their regulation of phytoplankton community composition, frequency of toxic/harmful algal blooms, habitat quality for metazoans, reproduction/growth/survival of pelagic and benthic invertebrates, and subtle changes such as shifts in the seasonality of ecosystem functions Each aspect of the Phase II model is illustrated here with examples from coastal ecosystems around the world In the last section of this review I present one vision of the next (Phase III) stage in the evolution of our conceptual model, organized around 5 questions that will guide coastal science in the early 21st century: (1) How do system-specific attributes constrain or amplify the responses of coastal ecosystems to nutrient enrichment? (2) How does nutrient enrichment interact with other stressors (toxic contaminants, fishing harvest, aquaculture, nonindigenous species, habitat loss, climate change, hydro- logic manipulations) to change coastal ecosystems? (3) How are responses to multiple stressors linked? (4) How does human-induced change in the coastal zone impact the Earth system as habitat for humanity and other species? (5) How can a deeper scientific understanding of the coastal eutrophication problem be applied to develop tools for building strategies at ecosystem restoration or rehabilitation?

https://www.int-res.com/articles/meps/210/m210p223.pdf

Our evolving conceptual model of the coastal eutrophication problem

The way that nutrients cycle through atmospheric, terrestrial, oceanic and associated biotic reservoirs can constrain rates of biological production and help structure ecosystems on land and in the sea. On a global scale, cycling of nutrients also affects the concentration of atmospheric carbon dioxide. Because of their capacity for rapid growth, marine microorganisms are a major component of global nutrient cycles. Understanding what controls their distributions and their diverse suite of nutrient transformations is a major challenge facing contemporary biological oceanographers. What is emerging is an appreciation of the previously unknown degree of complexity within the marine microbial community.

/pdf/marine-microorganisms-and-global-nutrient-cycles-1dwx3j7j0q.pdf

Marine microorganisms and global nutrient cycles

Prediction of the impact of global climate change
on marine HABs is fraught with difficulties. However,
we can learn important lessons from the fossil
record of dinoflagellate cysts; long-term monitoring
programs, such as the Continuous Plankton Recorder
surveys; and short-term phytoplankton community
responses to El Nin˜o Southern Oscillation
(ENSO) and North Atlantic Oscillation (NAO) episodes.
Increasing temperature, enhanced surface
stratification, alteration of ocean currents, intensification
or weakening of local nutrient upwelling,
stimulation of photosynthesis by elevated CO2,
reduced calcification through ocean acidification
(‘‘the other CO2 problem’’), and heavy precipitation
and storm events causing changes in land runoff
and micronutrient availability may all produce contradictory
species- or even strain-specific responses.
Complex factor interactions exist, and simulated
ecophysiological laboratory experiments rarely allow
for sufficient acclimation and rarely take into
account physiological plasticity and genetic strain
diversity. We can expect: (i) range expansion of
warm-water species at the expense of cold-water species,
which are driven poleward; (ii) speciesspecific
changes in the abundance and seasonal
window of growth of HAB taxa; (iii) earlier timing of
peak production of some phytoplankton; and (iv)
secondary effects for marine food webs, notably
when individual zooplankton and fish grazers are differentially
impacted (‘‘match-mismatch’’) by climate
change. Some species of harmful algae (e.g., toxic
dinoflagellates benefitting from land runoff and ⁄ or
water column stratification, tropical benthic dinoflagellates
responding to increased water temperatures
and coral reef disturbance) may become more successful,
while others may diminish in areas currently
impacted. Our limited understanding of marine ecosystem
responses to multifactorial physicochemical
climate drivers as well as our poor knowledge of the
potential of marine microalgae to adapt genetically
and phenotypically to the unprecedented pace of
current climate change are emphasized. The greatest
problems for human society will be caused by being
unprepared for significant range expansions or the
increase of algal biotoxin problems in currently
poorly monitored areas, thus calling for increased
vigilance in seafood-biotoxin and HAB monitoring
programs. Changes in phytoplankton communities
provide a sensitive early warning for climate-driven
perturbations to marine ecosystems.

/pdf/ocean-climate-change-phytoplankton-community-responses-and-3vq5zutr9a.pdf

Ocean climate change, phytoplankton community responses, and harmful algal blooms: a formidable predictive challenge

Ongoing climate change is predicted to affect individual organisms during all life stages, thereby affecting populations of a species, communities and the functioning of ecosystems. These effects of climate change can be direct, through changing water temperatures and associated phenologies, the lengths and frequency of hypoxia events, through ongoing ocean acidification trends or through shifts in hydrodynamics and in sea level. In some cases, climate interactions with a species will also, or mostly, be indirect and mediated through direct effects on key prey species which change the composition and dynamic coupling of food webs. Thus, the implications of climate change for marine fish populations can be seen to result from phenomena at four interlinked levels of biological organization: (1) organismal-level physiological changes will occur in response to changing environmental variables such as temperature, dissolved oxygen and ocean carbon dioxide levels. An integrated view of relevant effects, adaptation processes and tolerance limits is provided by the concept of oxygen and capacity-limited thermal tolerance (OCLT). (2) Individual-level behavioural changes may occur such as the avoidance of unfavourable conditions and, if possible, movement into suitable areas. (3) Population-level changes may be observed via changes in the balance between rates of mortality, growth and reproduction. This includes changes in the retention or dispersion of early life stages by ocean currents, which lead to the establishment of new populations in new areas or abandonment of traditional habitats. (4) Ecosystem-level changes in productivity and food web interactions will result from differing physiological responses by organisms at different levels of the food web. The shifts in biogeography and warming-induced biodiversity will affect species productivity and may, thus, explain changes in fisheries economies. This paper tries to establish links between various levels of biological organization by means of addressing the effective physiological principles at the cellular, tissue and whole organism levels.

/pdf/climate-change-effects-on-fishes-and-fisheries-towards-a-1sfdj8i5bb.pdf

Climate change effects on fishes and fisheries: towards a cause-and-effect understanding

[1] An overview of the first spatially explicit, multielement (N, P, and C), multiform (dissolved inorganic: DIN, DIP; dissolved organic: DOC, DON, DOP; and particulate: POC, PN, PP) predictive model system of river nutrient export from watersheds (Global Nutrient Export from Watersheds (NEWS)) is presented. NEWS models estimate export from 5761 watersheds globally as a function of land use, nutrient inputs, hydrology, and other factors; regional and global scale patterns as of 1995 are presented here. Watershed sources and their relative magnitudes differ by element and form. For example, anthropogenic sources dominate the export of DIN and DIP at the global scale, although their anthropogenic sources differ significantly (diffuse and point, respectively). Natural sources dominate DON and DOP export globally, although diffuse anthropogenic sources dominate in several regions in Asia, Europe and N. America. “Hot spots” where yield (kg km−2 yr−1) is high for several elements and forms were identified, including parts of Indonesia, Japan, southern Asia, and Central America, due to anthropogenic N and P inputs in some regions and high water runoff in others. NEWS models provide a tool to examine past, current and future river export of nutrients, and how humans might impact element ratios and forms, and thereby affect estuaries and coastal seas.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2005GB002606

Sources and delivery of carbon, nitrogen, and phosphorus to the coastal zone : An overview of global nutrient export from watersheds (NEWS) models and their application

Anthropogenic loading of nutrients in rivers often increases disproportionally among N, P, and Si, and thus may shift the type of phytoplankton nutrient limitation in the coastal receiving waters. The effect of anthropogenic nutrient loading has rarely been addressed in the Pearl River estuary along the southern coast of China, even though it is one of the largest rivers in the world. We conducted a cruise along the Pearl River estuary and adjacent coastal waters south of Hong Kong during July 17 to 18, 1999. Samples were taken for salinity and nutrients (NO3, SiO4, PO4, NH4 and urea) and nutrient addition experiments were conducted on board. Vertical profiles of salinity showed a salt-wedge estuary and the coastal plume covering the waters south of Hong Kong. Con- centrations of NO3 were very high (ca 90 µM) upstream of the Pearl River estuary, and much of the riverine NO3 was not utilized in the estuary until depletion at the edge of the coastal plume on the east side of Hong Kong. SiO4 was 120 µM upstream and its utilization was similar to that of NO3. PO4 was low in surface waters (<0.5 µM) and higher below the halocline in the estuary. NH4 and urea were generally < 4 and 1.5 µM, respectively. In the estuary, N:P ratio was 200:1, indicating potential P limitation, while N:Si was below 1:1. Beyond the coastal plume to the east of Hong Kong, N:P and N:Si ratios were <5:1 and 1:0.3, respectively, indicating potential N limitation. Nutrient limitation was shown in nutrient addition experiments and was consistent with the ratios of nutrients. There- fore, nutrient limitation shifted across the coastal plume from P limitation in the estuary to N limita- tion in the oceanic waters. Potential P limitation was observed in the estuary; P and Si co-limiting occurred at the edge of the coastal plume, and N was limiting in the oceanic side. This spatial shift in nutrient limitation has great implications for nutrient pollution control and coastal management of Hong Kong waters.

/pdf/shift-from-p-to-n-limitation-of-phytoplankton-growth-across-45b7fpaph7.pdf

Shift from P to N limitation of phytoplankton growth across the Pearl River estuarine plume during summer

The Pearl River in the south of China is the second largest river in China in terms of discharge volume. Two cruises were made to investigate the dynamics of nutrients and phytoplankton biomass in June and July 1998, across the Pearl River estuary to the adjacent territorial waters of Hong Kong. On-deck incubation experiments of 5 mixtures of freshwater from the surface with seawater from below the halocline were conducted to simulate time scales of phytoplankton blooms for each freshwater/seawater mixture and to examine uptake of nutrients. In July, phytoplankton growth rates increased with salinity of the mixtures, with the lowest growth rate (0.81 d -1 ) in freshwater and the highest (2.41 d -1 ) in 100% seawater (salinity = 29). PO 4 was lower in freshwater (0.3 μM) than in seawater (1.2 μM), whereas concentrations of NO 3 +NH 4 +urea (-80 pM) and SiO 4 (150 pM) were higher in freshwater than those in seawater (25 pM for nitrogen and 26 pM for SiO 4 ). During the incubation PO 4 disappeared first, indicating that P was limiting the phytoplankton biomass. All mixtures reached the maximum in phytoplankton biomass in 3 to 4 d. There was a regional maximum of phytoplankton biomass that occurred at the seaward edge of the estuarine plume during June. The region moved eastwards (away from the estuary) to the southern waters between Lantau Island and Hong Kong Island during July. Mixing diagrams of NO 3 and SiO 4 showed conservative behaviour with salinity in the estuary. These observations suggest that dilution by freshwater outflow was a controlling factor in determining the distribution of nutrients and phytoplankton biomass in the estuary due to high flow during June and July. The regional maximum of phytoplankton biomass was comparable to that resulting from the incubation and coincided with the exhaustion of PO 4 during July. On the estuarine (west) side of the regional maximum, chl a fluorescence increased during 24 h incubations, but decreased at the station with the maximum and on the east side, suggesting the possible limitation of nutrients to the phytoplankton community. In the same eastern waters, both PO 4 and SiO 4 were very low. However, NO 3 and NH 4 remained abundant, suggesting possible co-limitation by phosphorus and silicon. We hypothesize that considerable seasonal rainfall in June and early July might have contributed an additional source of nitrogen to the water column, which resulted in the exhaustion of PO 4 and SiO 4 before nitrogen.

/pdf/dynamics-of-nutrients-and-phytoplankton-biomass-in-the-pearl-3dk31zuy1p.pdf

Dynamics of nutrients and phytoplankton biomass in the Pearl River estuary and adjacent waters of Hong Kong during summer : preliminary evidence for phosphorus and silicon limitation

The magnitude of El Nino in 1997-98 was one of the strongest of the century A series of red tides occurred in Hong Kong territorial waters between mid-March and mid-April 1998, resulting in the loss of HK$250 (US$32) million in fish kill damage. The causative species was Gyrodinium aureolum. We used a time series of satellite images to track the development of the harmful algal bloom and relate its movement to physical oceanographic conditions. Prior to the red tide event in Flong Kong, harmful algal blooms had occurred earlier (November 1997-February 1998) along the south China coast to the east of Hong Kong, although the species were different. The progression of harmful algal blooms front northeast to southwest coincided with the southwesterly movement of the south China Coastal Current as indicated by the satellite images of SST (sea surface temperature) from the satellite AVHRR, Remote sensing images of chlorophyll a (chl a) from SeaWiFS (sea-viewing wide field of view sensor) confirmed high near-shore chl a for the same region, The entire event coincided with the dramatic change in the oceanographic conditions of the northern portion of the South China Sea between 1997 and 1998 for the period of March to mid-April, as revealed in the weekly composite SST. The SST images showed a warm tongue pointing north to the south China coast in 1998 versus a cold tongue pointing south in 1997 in the middle of the South China Sea. The differences are believed to be due to El Nino and responsible for setting up the physical oceanographic conditions which were favorable for the formation of harmful algal blooms along the south China coast. The warm tongue in SST suggested that the warm water from the South China Sea might have been piling up towards the south China coast. On the other hand, downwelling of the south China Coastal Current along the coast due to the Northeast Monsoon during March might have been moving against the South China Sea warm water at the bottom. As a result, the coastal waters of the south China coast including Hong Kong became trapped along the coast. Given local eutrophied conditions of the China coast, the outbreak of harmful algal blooms occurred over a coast-wide scale (∼400 km) in winter 1997 and spring 1998. It appears that the use of satellite SST images is helpful in detecting large-scale changes in oceanographic conditions and relating the changes to possible outbreak of harmful algal blooms.

/pdf/red-tides-during-spring-1998-in-hong-kong-is-el-nino-2iwuopf06r.pdf

Jay Chung Chen

Papers

Shift from P to N limitation of phytoplankton growth across the Pearl River estuarine plume during summer

Dynamics of nutrients and phytoplankton biomass in the Pearl River estuary and adjacent waters of Hong Kong during summer : preliminary evidence for phosphorus and silicon limitation

Red tides during spring 1998 in Hong Kong:is El Niño responsible?