Status of shallow water oxygen indicator development in

Stella-Theresa Stoicescu, Urmas Lips Tallinn University of Technology Estonia Analyzed stations / data

• Monitoring data from 2010-2018 from May to October were analyzed • Stations in the sea areas without the permanent halocline but seasonal thermocline were selected • Basins – Narva Bay, Tallinn Bay, west of island (Baltic Proper, formally EGB) and • Time-series of near-bottom oxygen concentrations • Sampling 6 times a year (here used data from 4 Cruises: late May-early June, July, August, and October) 2 Narva Bay and Baltic Proper

• Hypoxic conditions in the Narva Bay catched only once • Measurements 4 times in May- October do not show shorter periods with hypoxia • None hypoxia events registered at station 34a • Only once the concentrations were below 4 mg/l

3 Gulf of Riga

• Hypoxic conditions have been observed in the central Gulf of Riga quite often during the last decade • But not in the shallower areas (depth <40 m) • Further analysis conducted (presented later)

4 Indicator

• Method for finding the indicator value is under development. For Assessment area Station minimum Result the status assessment, yearly of median values minimum values from a of yearly minima monitoring station (gathered from Gulf of Finland (areas 5.2 mg l-1 GES May/June to October/November) without the halocline, e.g. off-shore areas of within the assessment period, e.g. Narva Bay) 5-6 years, should be pooled and Northern Baltic Proper 5.2 mg l-1 GES the median value calculated. If (areas without the even one of the stations median halocline) value is below the GES threshold Gulf of Riga (off-shore 2.4 mg l-1 Non-GES then GES is not achieved. area)

5 Further development

The GES value of 4 mg/l is proposed, considering the following: • Hypoxic conditions in the can be developed naturally and the bottom fauna is adapted to low oxygen conditions; • The oxygen concentration in seasonally stratified sea areas should remain above 4 mg/l.

However, • GES threshold could be different for different sea areas depending on the benthic species and the oxygen concentrations required for their healthy life cycle. • Oxygen concentrations in the near-bottom layer of sea areas with the seasonal thermocline decrease from spring to autumn. The decrease could depend on hydrography of basins (e.g. water exchange with the adjacent areas, separated with a sill or not, etc.) 6 Investigating hypoxia in the Gulf of Riga

Extensive hypoxia in the Gulf of Riga in 2018 – an exceptional or regularly occurring phenomenon (Stella- Theresa Stoicescu1, Jaan Laanemets1, Taavi Liblik1, Maris Skudra2, Oliver Samlas1, Inga Lips1, Urmas Lips1) 1 – TalTech; 2 – Latvian Institute of Aquatic Ecology Gulf of Riga - background

• Semi-enclosed shallow basin in the Baltic Sea (homogenous water column in the winter) • Area = 16 330 km2, volume = 424 km3, mean depth = 26 m; Ruhnu Deep 56 m • Water exchange with the Baltic Sea: • Irbe Strait (70-80%, sill depth of 25 m, cross section area 0.4 km2) • Suur Strait (sill depth of 5 m, cross section area 0.04 km2) • Mean runoff 36 km3 year-1: Daugava (70%), Lielupe, Gauja, Pärnu & Salaca • Water renewal period ~3 years • High inputs of N (90 544 t year-1) and P (2 427 t year-1) (2017) – higher than MAI (BSAP) (HELCOM, 2019) • Increasing P pool from 1974 till mid-1990s, afterward no clear trend (Yurkovskis, 2004, HELCOM, 2018) Ojaveer, 1995; HELCOM, 2002; Stiebrins and Väling, 1996; Petrov, 1979; Berzinsh, 1995; • P pool largely governed by internal processes, river input <15% of pool Yurkovskis et al., 1993; Johansson, 2016; (Yurkovskis, 2004) Omstedt et al., 1997; Lilover et al. (1998) 8 Station data, where depth >= 40 m DO trend -0.45 mg l-1 year-1 Historical data (Oct-Nov, n = 13, R2 = 0.50, p < 0.05) Phosphate conc trend: 0.08 μM year–1 Available data from (Aug, n = 14, R2 = 0.47, p < 0.05) regular Estonian and Latvian monitoring and research cruises at the central, deep stations G1 and 121 in 2005–2018 showed high variability of oxygen concentration in the near-bottom layer. Hypoxia: DO ≤ 2.9 mg l–1 KESE, ICES/HELCOM, SeaDataNet, LV environmental databases9 Vertical profiles for estimating the extent of hypoxia Time series of the vertical distributions of temperature, salinity, density anomaly, and oxygen concentration at monitoring stations G1/121 in 2012–2019.

10 Estimates of the hypoxic area in GoR: 3.8% 1.7% 1.4% 4.5% Section from open Baltic to Daugava river in 2018 32 114 G1

late May mid July late August 2018 11 Oxygen consumption estimates

풄풐풏풔풖풎풑풕풊풐풏 풕ퟐ 풕ퟐ∗ • 푶ퟐ = 푶ퟐ 푮ퟏ − 푶ퟐ (푮ퟏ) / 풏, where

푐표푛푠푢푚푝푡푖표푛 푂2 – monthly oxygen consumption in the deep layer; 푡2 푂2 – expected oxygen concentration at station G1 at time t2; 푡2∗ 푂2 – measured oxygen concentration at station G1 at time t2; n – time between two measurements (t1 and t2) in months. 푺풂풍풕ퟐ(푮ퟏ)−푺풂풍풕ퟏ(푮ퟏ) • 푶풕ퟐ(푮ퟏ) = 푶풕ퟏ(푮ퟏ) + 푶풕ퟏ ퟏퟏퟒ − 푶풕ퟏ 푮ퟏ ∗ , where ퟐ ퟐ ퟐ ퟐ 푺풂풍풕ퟏ(ퟏퟏퟒ)−푺풂풍풕ퟏ(푮ퟏ)

푡1 푂2 – measured oxygen concentration at time t1 at station G1 or 114 푆푎푙 – measured salinity at time t1 or t2 at station G1 or 114

Oxygen depletion and the estimated consumption values were higher in 2018 (mean consumption per unit volume of the near-bottom layer -2.7 mg l-1 month-1) than in 2017 (mean -1.2 mg l-1 month-1).

The estimated mixed near-bottom layer thickness was in 2018 about two times smaller than in 2017.

The consumption rates per unit bottom area were in the same range in 2017 and 2018, varying between 1.1 and 1.8 mmol m-2 h-1. 12 GoR historical and 2018 data analysis results

• Continuing decreasing trend in oxygen concentrations since 2005 in the Gulf of Riga near-bottom layer and occasional hypoxia events. • Hypoxic conditions develop in the near-bottom layer if stratification (thermal and haline) is strengthened, hindering vertical mixing. • Based on 2018, a sequence of certain processes contributed to the development of hypoxia • Large river runoff in the of 2017 and beginning of 2018 raised nutrient and organic matter concentrations • Early stratification development – in spring, the water column is usually homogenous, but in April 2018 there were salinity and DO gradients in the deep layer; in March inflow (from the Irbe Strait) favouring winds (E-ENE) • Strong stratification – inflow favouring winds in March(E), May (ENE), July (NNE); higher than average air temperature • Since mid-August almost no lateral transport into the deepest part of the gulf (no inflow favourable winds & deepening of the thermocline) -> developing hypoxia 13 High frequency data to study the confidence of estimates

Time series of (A) wind speed, and vertical distribution of (B) temperature, (C) salinity and (D) oxygen measured at buoy station from 5 to 21 August 2018 (close to station G1).

DO = 2.9 mg l-1 marks the hypoxia threshold.

Estimates of the extent of hypoxic areas could vary considerably when using a single profile

Also consumption estimates vary 14 Assessing the status and confidence

• High frequency data – high oxygen variability • Regular monitoring data – capturing seasonal course of parameters

• Regular monitoring data can be used for consumption estimates, when taking into account the variability of results. Confidence bounds can be determined, based on high frequency data, as the standard error of the -2 -1 results (0.26 mmol O2 m h in the presented example). 15 Conclusions

• Oxygen levels below a set threshold (e.g. 4 mg/l) could be used or species-specific values could applied • The extent of hypoxic bottoms could be used • Such seasonal minima or maxima can be applied if data with good enough resolution are available • Consumption estimates for basins with sills can be applied • High-frequency measurements are essential (also, for estimating confidence of assessment results) • Numerical models could be used for extrapolation (not for stand- alone use yet)

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